First of all, the game is wonderful. It's a worthy successor to SSX Tricky; if you liked its predecessor, you'll like it too. I'll probably do a more thorough review at some point, but a few things for now:
1. As far as I can see, none of the original levels survive. If they exist, they'd need to be accessed via a cheat or something. The concept of Merqury City survives in the Metro City Breakdown, but that's it.
2. In fact, the game has been completely restructured. In SSX 3, all the events take place on one mountain, rather than being scattered across the world as in Tricky. The mountain has three peaks, each of which is more difficult than the other. To unlock each peak, you have to defeat all the race or all the trick levels of the peak before it.
It's not that the opponents get more difficult, although they do to some degree; it's that the mountain gets more difficult. Peak 2 has several places where wind tries to push you around or slow you down, and in one of its courses lightning knocks down trees right in front of you. Peak 3 is truly scary--entire sections of course fall away as you're riding. At one point, you can grind on the wreckage of a downed jumbo jet. The trick on peak 3 seems to be to not plan ahead, because if you do, conditions will change and you'll fall into some sort of deathtrap. There's a reason the game's tagline is "it's you against the mountain".
3. There are numerous enhancements to the trick system. The game adds ubergrinds and presses (manuals to Tony Hawk fans). Ubertricks are now true grabs, and you can hold them as long as time permits; this looks a bit weird on tricks like the Pirouette Grind or Walking The Dog, but it's nice for racking up points. Tricks that come in sufficiently close intervals are considered to be combos, even if you landed between them; this means that in the Superpipe event, most of your tricks are linked into one giant combo, so you end up with an 8x multiplier or so if you don't fall. There's also a recovery system--basically, if you hit B fast enough, you can recover from a midair collision, or salvage most of your boost. The ubertrick system has also changed a lot--I'll probably go over that at some later point.
I'm currently on peak 2 with Kaori, working on the second of its two normal race courses. Last I checked, my brother was working on peak 3, but he was planning to pull an all-nighter and the last I heard from him was last night, so that might have changed.
Well, back to gameplay...