OmniFocus 3 for Mac should offer to import the OmniFocus 2 Archive the first time you archive there, so I think if you do that before removing OmniFocus 2 you should be good to go. We’ve been thinking about ways to make this a little more obvious.
That depends on how you uninstall the app. Just deleting the .app bundle from /Applications/ won’t delete the archive.
The archive file is named Archive.ofocus-archive and lives alongside the default OmniFocus.ofocus database in the OmniFocus container:
In OmniFocus 3, there’s no title bar and thus no proxy icon. But if you want to quickly get to the OmniFocus Application Support directory, hold option key and open the File menu. You should see the item Show “<document name>” in Finder, which will do the same thing.
I was afraid to perform an archive from OF3 because I was afraid of having a separate archive database. But based on your comments I went ahead and did it, and sure it enough it offered to import my OF2 archive so I think I’m good to go.