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.
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press http://www.applemotion.net
Just was concerned that the two apps (Motion 3 & 5) would work ok on the same system really. I knew about the round tripping thats not a problem right now. I'm just slowly transitioning from FCS2 to FCP X Motion 5 and I have a ton of old projects that I need to access so being cautious and making sure my system(s) keep running normally.