Bottom line... install X3 first, then X3b...
... and yes, during the install you can elect to install thing to different places...
I have a 240Gb SSD as my main (O/S) drive. Data I often put on another (7200rpm) drive. There "is" a glitch that I found in that X3 made a project folder on my C: drive... even though X1/X2 was assigned to another. In this case, you can re-assign under preferences (in X3)... and *copy*, NOT move those to the other drive... *verify* they work (please), then remove the C: copy.
In ALL cases, protect the integrity of anything you have done (copy elsewhere, etc.). I like the SSD because of its speed, but it *has* had errors/issues (for me) in the past, so I put "data" on a magnetic HD. I used to design/develop for Seagate, so am not confident in SSDs (yet)...
Michael