PS3 is incredible value now -- it wasn't when I originally bought one. Now its cheap, it looks good, it has a great library, and its probably got the best value of any home console out right now, especially if you get PSN to pick up the freebies as well. Having said that, as you already have a 360, I'd say its only really worth it if you haven't played the big Sony exclusives yet, or if you're the type of dude excited by the recent Persona announcement etc. PS4 isn't backwards compatible though, so if you've missed out on the likes of Last of Us and other such content, its not a bad investment.
3DS would probably complement what you already have, give you some different experiences, but its doing well enough that the price has remained quite high for what it is. I'd definitely recommend getting one at some point as it has some really must-have games.
Maybe when you're definitely done with your 360 and moving on, you could trade it towards something else like these...