It is not clear, from reading your post, whether or not you already have X3 installed.
If not, then I suggest downloading and installing the demo version of both the 32-bit and 64-bit versions of X3.
With X3 installed, go ahead and try installing and testing that plugin within X3 64-bit. It will either: fail completely, partially work with some glitches, or will work just fine.
In any case, it will not hurt anything to try. Worst case, it doesn't work in X3 64-bit.
Then, test is in 32-bit X3, and figure out if it works there. Again, worst case is that it doesn't, but it likely will run just fine in a 32-bit Sonar X3.
So, after testing in both, you should know where things stand, with regard to where you can use that plugin.
If it does not work, it will not 'break' Sonar, even if using it in Sonar crashes Sonar. If it does, then restarting Sonar will fix Sonar - just then don't use that plugin in Sonar, and Sonar will be just fine.
On MY system, when I was still using 32-bit plugins in my 64-bit Sonar, I created a sub-folder within my 32-bit VST Plugins Folder, called 64-bit Tested Plugins, and when I was satisfied that a particular 32-bit plugin worked in my 64-bit Sonar, I would move that plugin into the new sub-folder. By ONLY including that sub-folder in the search path for my 64-bit Sonar, rather than including its parent folder (with the rest of the 32-bit plugins), I then only 'saw' 32 bit plugins that I KNEW would work in 64-bit Sonar.
Bob Bone