Really creative. I thoroughly enjoyed the mixes. I have no useful advice on plugins. For me, I pick kinda randomly. I know a certain set of plugins pretty well and I tend to use them a lot.
 
One that I regularly use is MeldaProductions M-AutoAlign. It's useful for calculating offsets for mics and reducing phasing issues. It's kind cool. I know where the mics were in the room, and it often exactly calculates the number of feet between the mics (or fairly close anyway) as shown in the milliseconds of offset.

I think Ozone is a good buy as regards "Bang for buck". I also like a lot of the PSP stuff as well, but Ozone has it "all in one place". They have a demo available that'll give you an idea of it before splashing out for it. 

Ozone 6 .............coming soon ;)
 

I was in a similar situation with the OP a year ago and after my searching, I went with the Christmas deal on iZotope's "Studio Bundle" (Nectar 2 (vocals), Alloy 2 (mixing), Ozone 5 (mastering)). In 15 years, this was the first time I had purchased a non-Cakewalk plugin. To reiterate comments made above, I think the best feature of these is that they are essentially "stand-alone effect chains," which fits well with Beepster's minimalist comment (and is a personal pursuit of mine as well). I had done a bit of research on options prior to this purchase, and made full use of their 10-day trial prior to buying them. Interestingly, they have features in them that are not spelled out in their manual, and I found by randomly clicking around the UIs; but the plugins themselves are very capable and tweakable, and allow for saving as presets to enable faster workflows with each use. Ironically, there was a thread about FabFilter Pro-Q shortly after I made my purchase and I saw a tutorial on mid/side EQ... at that time I thought "Oh well, I have already committed to iZotope," but turned out that iZotope plugins have this capability as well. Overall, I think these carry very high "bang for the buck" value with them.
 
As an aside, iZotope also has free guides posted on their website. These are written at a more "global" level, so can be applied to any similar plugin, but of course the details are focused to their specific products. The Alloy 2 guide has nice mixing tips, and the Ozone 5 guides have mastering advice and tips on dithering. https://www.izotope.com/en/support/support-resources/guides/

Karyn
I guess I should point out at this stage that if the conversation goes to much towards 3rd party software then it should really be in the Software forum.  That is what it's for after all...
 
Just saying.

given the thread title, how could it not be?

None of us here are really discussing Sonar. It's just been mentioned a few times, so the thread probably should be moved. Anyway please carry on don't want to hijack the thread with talking about how it should be managed. Otherwise might be moving it to forum discussion forum :)

ok,  I tried to keep it X3 related by suggesting the OP tried out more of the bundled stuff and the additional PC plugs,  but to save further argument, it's being moved.

Boydie
...I was looking for plugins to COMPLIMENT the stock X3 plugins...

With that specific focus in mind, what you want to be looking at are things that aren't already bundled with SONAR -- NOT things that may or may not do the same stuff better. Fortunately, you really do already have - literally - everything you need in X3. Not just blowin' smoke; you really do.
 
That wasn't always the case. For a long time SONAR didn't have an opto-style compressor or a good auto-release limiter and everybody had to go third-party. Today, though, every basic category is covered. Not just adequately, but well-covered and perfectly capable of making professional-quality records. 
 
So what's X3 still missing? I can think of only two things: a dynamic equalizer and a really good harmonic exciter. The best of the dynamic EQ category would be either MDynamicEQ or MAutoDynamicEQ from Meldaproduction. Both have been on sale multiple times this year. For harmonic exciter, SDRR from Klanghelm is good and cheap. But if you go with Ozone, it includes an excellent exciter, negating the need to buy another one.
 
Third-party mastering tools remain popular, but mostly for ease-of-use - not because they're drastically "better". Ozone would top the list of plugins that don't do anything new but are just easier to get good results with. But it's largely a matter of personal taste. Ask for recommendations and you'll get as many opinions as there are users for each of the popular products. Truth is, all of them do just fine. But Ozone is far and away the easiest to use.
 

Ozone & Waves +1 ...consolidated interface. Ozone starts you out with some workable presets, but you'll want to adjust them to your mix for optimum sound. Izotope offers a free online publication to cover the basics of mixing a master and some of their ideas on mixing approaches.
 
Honestly in a few cases I have used The basic plug-ins in Sonar X-3 and have gotten the same or better results as I did using Ozone. Don't forget that Sonar X-3 has pre built effects plug in chains for both channel and master work. So I don't necessarily always use Ozone, but if I were to steer you to something that can be an additional help that would be it. 
 
Some Waves plug-in are better than others. They tend to have a high level of quality though overall. I think you can download a free demo to try their stuff out. Good luck!

When it comes to plugins, the best choices come down to personal preference.  Sound quality differs, but these days you can achieve good sonic results from just about anything if you work with it enough.  So, other factors like ease of use, stability, CPU usage, support policies, and price are more relevant factors.  And these likely will differ from user to user, so demoing the tools that interest you on your system with your material is a must!
 
The real benefit to third-party tools comes in when something about them clicks with your personal tastes and workflow.  If you really enjoy using a particular tool, you're likely to get your money's worth out of it, since you aren't struggling with poor GUIs or glitches and can focus on more creative matters. 
 
Now, there are cases where you want something specific and can't really achieve it with stock plugins.  Say, a lush Lexicon reverb sound or an authentic 1176 smack on your snare drum.  Third parties to the rescue in these cases, but again, it won't matter much unless you know how to use the tools. 
 
For me, the bottom line is that if a tool comes along that clearly improves on my existing stuff in some way, and it's priced fairly, then it's a good purchase.  Plugin technology is constantly improving, and is a fascinating industry.  Lots of fun to watch the market and play with new toys ... and sometimes buy them.   
 
Oh, and Ozone / Alloy are fine, but iZotope plugs aren't known for being CPU-friendly, so a demo is important to see if they'll actually be usable for you.  The Waves Gold bundle is also very comprehensive and a good value when on sale.  The ToneBoosters line is probably the best bang-for-buck suite on the market.  Higher-end tools from the likes of FabFilter, UA, and HOFA are worth looking into if your budget can take it.  Anyway, good luck and have fun! 
 
Cheers,
Eddie
 

CakeAlexS
It's very much an individuals choice IMHO on where you end up spending the money... I would subscribe to Groove3 myself.

That's a most excellent piece of advice.  And if they (Groove 3) are true to form, you'll be able to get an annual full access one-year pass in Nov. for 99 bucks.  I'll be signing up for the third year straight if they deliver on the special price once again.
 
The other advice is to just be patient and watch the marketplace.  I've gotten some outrageously good bundle prices in the occasional group buy or special sale.  I picked up everything (at the time) in Nomad Factory's catalog for a hundred bucks a few years back.  There was a recent sale of the Plug & Mix VIP bundle for 49 bucks recently, if I'm not mistaken.  Now, not everything in either of these packages is useful or even very good.  But there are some gems in both, and for the price, these were wonderful buys.  But don't be in a hurry.  What came with SONAR should keep you happily mixing for the moment.

You can get great deals on Ik Multimedia's T-racks Deluxe at sites like KVR. I got it for $69 and all processors work as single plugins. I would suggest you buy Cakewalk's Pro channel stuff, they are very good IMHO. Fab Filter's mastering bundle is very nice, but If your on a budget look at Toneboosters, Klanghelm and Hornet. For a very nice Limiter try the free LimiterNo.6 and Loudmax.

Bit late to this discussion, sorry.
I have ended up with Melda SpectralDynamics because I was trying to use the LP64 as a dynamic EQ but it just wasn't quite cutting it. Also, the Waves DeEsser because it has a few more tweaks than the VX64 vocal strip. I've tried a few other Waves plugs and I have the free Melda plugs but I always seem to go back to Sonar's plugs and Pro channel, as everyone seems to agree, they do everything needed, very quickly and very well at that.
I don't have a dedicating mastering suite, but then again I don't think I have the listening environment or the need yet, I tend to outsource any songs that need mastering to an engineer that I have known for a long time and trust his ears and equipment better than my own. It's also good to get a fresh set of ears listening to my mixes because by the time I've finished with them I am too invested in their sound to make any drastic changes.
I guess I'm agreeing with those that are saying, look for a need and go from there.

Thanks to all

I decided to go for the Izotope Mix (Alloy 2) and Master (Ozone 5) bundle

I got a great deal and also got the free upgrade to version 6 of Ozone - although the biggest benefit is having both as I still find myself drawn to Ozone 5 but I will continue to play with 6

Alloy 2 is also really good - the transient shaper is a joy to work with and the EQ (like Ozone) is really transparent and sounds different from the Pro Channel EQ so I have a different option for EQ

All in all I am very happy!