All of it is possible, it just a matter of what you need to do.
Upon installing FCPX, it will move all of FCS3 in a folder called "Final Cut Studio".
This might also break some relationships in FSC3 such as "send to Motion" et al. they can only be partially restored.
You can also put FCPX/Motion/Compressor (if you buy the latter two) in their own folder and move the original apps back in to place and after install.
Here's the official Apple stance:
http://support.apple.com/kb/HT4722?viewlocale=en_US&locale=en_US
Jeremy
