Completely Remove Sonar X3?

You won't like what happens if you do that. Expect to lose somethings like R-Mix.
 
And things will get wonky and you'll probably have to re-install Platinum.

Perfect Space and Pentagon I are no longer bundled with SONAR.

I happen to agree with the general notion that prior releases are good to keep around, both from a component/content retention standpoint, and from a stability standpoint.
 
By component/content retention standpoint - I mean that some components, as pointed out above, may not be carried forward from one release of Sonar to a newer release.   This is a frequent occurrence, so I generally leave the prior releases out there.  They don't take up much space.  Also, I frequently have customized track templates - lots of those, and some lesser number of other custom content, such as project templates, etc., so I generally keep that stuff around, at least until I copy it into the new release's content folders.
 
And, by stability standpoint - I mean that once you install a newer release of Sonar, and THEN go back and delete the prior release, there is a really good chance the newer release will start behaving strangely, and might even start crashing.  There are shared things between older and newer releases, and while a newer release is expected to be able to open an older release's projects, removing that older release should likely be done prior to installing the newer release, if you are meaning to just keep the newest release on your system.  One other aspect of stability I would mention, with regard to keeping the prior version installed - an additional reason I do so is that sometimes a new release may have some issues that will take some time to work through, where Cakewalk scrambles to create/test/release one or more hot fixes or other maintenance, or sometimes something the new release is doing doesn't work with some number of plugins that I rely heavily on - that sort of thing, and by keeping the prior release of Sonar installed, I then have the option of working on such projects in the earlier release and can remain nice and relaxed while waiting for whatever tweaks/fixes are needed are built and released.
 
Bob Bone