I don’t know what you mean by “I’ve tried changing the version .txt file to 16”, but that’s not going to work. To share projects using v.3 on the Mac, you need to be using the v. 3 Betas for windows, currently at RC 15, available here:
My Windows using collaborator has been using the Betas for the last two years with no problems other than user error by not waiting for sync’ing to fully complete. There are still things being ironed out.
Your other solution is to re-install Mac v. 2, which you can find on the website, or doing round-tripping by exporting to v. 2 from Mac 3; the problem with that is on every round-trip it creates a completely new version of the project, plus another back-up version every time you open it again on the Mac.
I’d use the Windows v. 3 Beta, but the choice is yours.
HTH
Mark