Are you guys like jailbroken or something? Or maybe using a ******* version? My brother has a 3g and it works fine for him. I have a 2g ipod touch and a 3GS and they it works fine on my 3GS and touch.
I don't really understand the complaints...It's obvious the game is a memory intensive app. It's an open world sandbox game for christ's sake! Turn off the things on the 3G that use up a lot of memory like Push notifications, restart the device, be smart. You can't expect something that big and powerful to not use anything to run, haha. Yes it's a 7 dollar game, but it's also packed full of lots of stuff.
If you have jailbroken iphone, just download SBSettings and run Safe Mode. Game runs much better for me, but it still have lags.