The buttons reduce friction between the two cams, and prevent the cams themselves from wearing out. The buttons also act as a spacer, moving the driven sheaves closer together (that is why your belt gets sucked down into the driven, although the spring could be bad too).
Yeas only 3 are required, the other three would be there if you made the cam face towards the engine, but the 30 series is asymmetrical so it only works with the cam outboard. The other three might also be used if you ran the system in reverse.
In short, you only need to use 3 buttons at any given point in time.