I can play xbox all day on some games with nearly no lag and others I can barely play at all because the lag is so bad. Basically, you'll want to try and stay away from certain games, specifically ones that play on a peer to peer connection unless you're playing in small groups like 4 on 4. Other games like Battlefield 3 for instance that use dedicated servers tend to play pretty well even on slower connections because it removes the middleman from the data connection. 3g was never really meant for gaming or for majorly extended use fort that matter however it can still be done.
This thread was really more of a PoC (Proof of concept) kind of post and I have made no guarantees that things will run well, just that they will run lol. Many factors go into your connection as well, time of day, signal level, network load, number of concurrent connections, physical distance to said connections. I'll use ( I know you're on a ps3 but I'm sure you've played halo online somewhere) Halo reach as an example here, change matchmaking settings to best connection and local only that way it will try to match you with low ping players within a shorter distance from you reducing the bounce between different servers to reach one another (see:
Traceroute).
As for getting better speed, reduce the amount of internet use going on, sign out of messengers, disconnect your computer (or if running from computer Internet connection sharing close any network processes). You can also try using the 3g turbocharger script that's posted somewhere here on the forums and try changing your PRL. Other than the above tips and points I can't really help you out much but I wish you good luck.