>open a file
What kind of file are you wanting to open?
I would very much recommend AGAINST trying to open a version 3 project with version 10. The program has evolved so much over the past 6 years it would lead to certain disaster -- or, at the very least, a corrupted project file.
I'm not sure what you mean by migrating all of your photos and MP3s -- but the Elements Organizer should find your current media catalog and update it just fine.
I am not sure whether 3.0 project will work fine in 10.0
There are lots of changes. Features have been added as well as removed. If you really want to try that, save a copy first and then try. but if it wont work it wont be unexpected.
The organizer is capable of updating all your media which are currently in your catalog. That would be a safe bet i suppose.