SONAR Platinum installation leaves X3 and older installed

I'd leave it if I were you; when I installed Platinum, I did. In fact I still have X1, X2 and X3 on mine.  They don't take up much space.  Also, if you do remove X3, there may be files (VSTs, instruments, etc) that Platinum relies on from the previous X3 installation. I guess you can remove specific files, but I don't think its worth the risk given the small overhead.
 
Jerry

The actual Sonar program folder doesn't take up a very large amount of space, all "content" is shared between versions and is by far the larger amount of data. Also, while Sonar generally is very good with backwards compatibility, sometimes a project won't play nice in a newer program version and it can be good to have the original version that created the project at hand. Also, uninstalling an older version could theoretically also uninstall some specific things that you got with it. E.g. X2 Producer had R-Mix and X3 didn't. You can solve this with a custom reinstall, but unless you really really need the small amount of space you'd gain it's easier to simply leave older versions.

If space on your system drive is at such a premium then it might be time to think about investing in a larger drive.
 
I agree with everyone who has said "don't uninstall previous versions"

You could always NTFS compress the old program folders - X2 goes from needing about 100MB to about 64MB.

Leaving old versions is by design. When I was really tight on space on an old XP box, I would zip the directories and move them to an external drive. This let me easily restore the old versions without having to reinstall. If I were to use the same strategy today I would archive "C:\Program Files\Cakewalk\SONARversion", "...Cakewalk Content\SONARversion", "C:\ProgramData\Cakewalk\SONARversion" and the "user\AppData\Roaming\Cakewalk\SONARversion." Of course this strategy does not account for data in non-default locations.
 
BTW, I have not looked at the total disk space used by an installation in some time, last time I looked was SONAR 8.5, my last version on XP and it was ~200MB. It has grown a bit due to the tutorials.
 

• The tutorial projects in Cakewalk Content are ~900MB.
• The rest of Cakewalk Content\SONARversion is ~200MB.
• C:\Program Files\Cakewalk\SONARversion is ~125MB.
• C:\ProgramData\Cakewalk\SONARversion is ~5MB.
• user\AppData\Roaming\Cakewalk\SONARversion starts out as a copy of ProgramData. I would point out that minidumps get stored in this directory. It may be a good idea to delete those before doing anything else.

 
So ignoring all the shared content (plug-ins, samples, utilities) a recent full installation consumes around 1.2GB. The largest user of space are the tutorials in Cakewalk Content.