Christian Svensson was on the Capcom Forums explaining why the Marvel vs. Capcom 2 demo lacks 3x kick and punch button configuration options. Here's his comment.
Why did you guys omit the 3x punch and kick buttons from the configuration for the Marvel vs Capcom 2 demo?
Sven: The game is built off of the original Dreamcast code and those button maps didn't exist in that version either. As such, it isn't an omission.