You need to stay under 10k vertices and 20 drawcalls for your game to have a chance of performing well on a 3G. I have a 3G as my primary test device and that's what I've seen.
The primary weakness of the 3G is its CPU is really slow so you won't be able to have much physics, AI, or any other CPU related activities going on.
This guide is written to help new users to Unity3D so that they can get started quickly creating games and compelling applications for the platform. by Guardius in How-To Guides/Manuals, Game, and newbie