You don’t need to uninstall Motion 3 in order to install Motion 5.
You should be able to open your Motion 3 projects in Motion 5 – but you can’t go the other way.
You can’t import Motion 5 projects into FCP7, nor can you roundtrip from FCP7 to Motion 5.
—
Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
https://www.applemotion.net