Hey guys, I managed to fix the Mac version of the game.

Just go into the game's files in steam (steamapps>common>grandtheftauto3 *Right Click* Show Package Contents).

Then go into Contents. Resources. Transgaming. c drive. program files. Rockstar Games. GTAIII.

Once you're there, delete (or just move) the folder called "movies".

Now you'll actually make it to the main menu!
Once there, use the arrow keys to go into settings. Turn on widescreen. And increase your resolution so the width is No more than 1650 (might be smaller, I guess it depends on the computer?).

Anyway, game should work. If you don't click widescreen and increase the resolution, nex time you open the game you might not be able to see anything. That's ok. Go to your Documents, Rockstar Games, GTA3 User Files, and delete gta3.set .

I hope this works for everyone else! Wish I'd found this out back before 64-bit Macs were on the rise...