Consider also that 1 and 2 were built using the same engine while Tony hawk 3 used a new engine built from the ground up. Entirely different physics, animations, everything.
Knowing that they are building this game on the source code from the classics, it makes sense to put these two alone in a bundle. They could certainly put levels from 3 in there and we would all have a great time, but the feel would be different.
I think we would all be better served by having a separate volume with the ps2 era games with their own physical rules. Better than the feel being different and we all complain that we can't use our muscle memory to complete these goals.