Haiii.
Okay, ive never played any of the skate games, but i count myself as a modern game guru since i read a LOT of reviews and stuff xD. but noones posted anything yet so i think i should just share some stuff ive picked up.
The jump between Skate 1 and Skate 2 is far more significant than 2 to 3; 3 does have a few improvements such as graphics and various gameplay things. However, its mostly regarded as a sort of Skate 2.5, and your better off either waiting for it to come down in price, or missing out on it totally. If your a HUGE fan of the Skate series, or staking games in general, you should pick it up but dont expect much.
In short, wait a while and get it when its cheap, if you can.
:]/