I never played the original SSX, only Tricky, so obviously I can't compare SSX3 to SSX. However, I can tell you that as much as I enjoyed Tricky, SSX3 is far better. The graphics have been spruced up, there are more events - racing, freeride, big air, etc...
You can also customise your character a lot more than you could in Tricky, instead of choosing the outfit, you can now choose each seperate item of clothing - gloves, hair, sunglasses/goggles, boots, board...
The music is great too, you can import your own Xbox soundtrack into the game, but I prefer to keep the music on the DJ mode. You're boarding along and the DJ says a few words about the course you're going towards, the weather conditions, or even the song he's about to play. Obviously it isn't real-time, but it still makes it feel a lot more realistic, something that I feel really gives the game a better feel.
I would strongly recommend buying SSX3 - it's a hard game, but really fun to play - and that's...