THPS1 was a game about combo lines (kickflip onto this rail, grind, 360 flip off into a melon grab before landing, etc) THPS2 let the player combo predefined lines together by the manual. THPS3 adds the revert which lets the player combo both street and vert lines together.
THPS3 is the culmination of the early Tony Hawk design. So much in fact that THPS4~ had to make substantial changes to the design in order to not be seen as limp-dicked retreads, but the changes were mostly for the worse.