I recently bought this game in hope of trying out the online.
Is it 2 or 3 that most people play?

Also, can I use the online via my PS3?

Well I think more people probably play 3 now but 2 is still the better game. The problem with SOCOM 2 now is that its infested with people using codes and cheats. But there are still a lot of genuine people who play it a few I know of here on GT so you could try and hit them up. And yes you can go online with it via your PS3 im pretty sure.