Multiple Monitor Use, I am looking for a 3 monitor setup. Has anyone used a 3 monitor setup with Sonar? What are the pros and cons,
Thanks!

There is only one "con" to multiple monitor setups.  That is, you will have chronic neck pain and eye strain if an ergonomic layout is not used. There are plenty of articles about this on the web including recommendations by OSHA.
I am sure that you have already had occasion to think when another monitor would have come in handy.  However, by asking about the pros and cons, it seems that you are asking about the value of the third monitor, the bang for the buck.
Once you have chosen an ergonomic layout, you can determine the cost of equipment and changing your work space. Cardboard mockups and tape outlines always helped me visualize space requirements. Now ask the piercing question "Will the third monitor help me make more money with my music?" or even "Can I use the money for a third monitor setup to buy another piece of audio equipment instead?"  Answer honestly and you will be happy with your choice.  

Well this doesn't answer your question but might get you thinking of another approach.

I have two monitors: a 27in 1080p monitor and a 34in Ultrawide. They both sit on my studio desk nicely so I didn't have to think about where to put a 3rd monitor. The ultrawide sort of counts as 1.5 monitors. 

I went from 2 1920x1080s to a large 4K, which is like 4 1920s 2 x 2 without gaps/frames.  One of my original 1920s is off to the side for reference docs and such.
 
 

I use a two-screen setup and it works pretty well. One gets the main screen, the other RME's Totalmix, plugin guis, help documentation etc. and the console view when I use it.

If I were to add another screen I'd probably consider a multi-touch one that can be laid out almost horizontally and use it as a mixer/console view surface, a job I currently use an iPad for. Or maybe next to a synth rack so I don't have to turn round to see the screen while playing.

Ergonomics are definitely something to be wary of with multi-screen setups, as are potential sound reflections from nearfield monitors off the screens or the screen's position making a less than optimum (audio) monitor placement necessary. You might also find that a couple of 4K or 5K screens work better than three "ordinary" ones from a placement point of view while providing a lot of display space.

In a somewhat related note. I have three monitors at work (not a DAW). One of the three is on a SVGA->USB adapter plugged into a USB port. Another is on the DVI port of the Vid card and the last one is a an SVGA->HDMI adapter that go into the same Vid card as the DVI. To be clear the DVI and HMDI ports are on a vid card not vid ports of the MB.
 
One problem I have is at least several times a week the 3rd monitor (USB) goes black. If I unplug the USB cable and plug it back in the 3rd monitor comes right back online.
 
This happens no matter what USB (2.0) port it's plugged in to or even if I swap physical monitors.

Of course, you've probably have already thought of this, but make sure your video card supports three if you go that route...