Well considering the fact that UT3 has a mod browser put right into the game, yeah, I'm pretty sure it's "legal" to mod the game. Plus, it's never a case where you can run a mod that someone else isn't running and have a leg up on them. When you are running a mod and someone else isn't, that's called hacking, which is different; and I'm not even sure that is "illegal" I don't think they would press charges, lol, you just might get your name slandered as a hacker and no one would want to play with you. But as for mods, there are some really cool ones in the browser, go check them out.
Yes they are legal and encouraged by the Unreal Tournament developers. They supplied the mod building tools for both PC and PS3 versions on their official website. Just keep in mind trophies are temporarily disabled when mods are in play. Once the game ends trophies are enabled once again. So nothing is permanent or harmful to gaining trophies.
No.. Many of the mods made for the PC never made to the PS3. The modders saw the ps3 as a waste of time. Or lost interest in it. Epic kinda dropped the ball and didn't release the UT3 script for the ps3 right away. They're are a ton available though all over the internets, easily transferable from pc to ps3 via flash memory stick.