Ok, so I've had an arch linux w/ gnome 3 installed for a while now. And a while ago I installed ubuntu as another partition, I think to fix an issue that cause arch to fail.
So, it was all working fine, but then I went and reinstalled grub 1 on the arch partition; Ubuntu had overwritten it on the install.
Then when I tried to boot into arch it booted, but the graphics wasn't working correctly: gdm wouldn't even show, and there were weird colors instead. So, I uninstalled xf86-video-ati and then installed xf86-video-vesa. That made gdm run in fallback mode and I was able to boot to gnome 3 fallback mode (or whatever it's called).
But I can't seem to get the graphics working correctly.