In the RBFF guidebook interview, the game's staff were asked about why they updated 'Real Bout' by changing the layout of the buttons. They responded:

"In Fatal Fury 3 we introduced the Oversway Line. It seemed to be very difficult for new players, but we were convinced the idea itself was good. So we tried to make it easier to use this game, which meant adding a dedicated button for it. That was the biggest reason for changing the entire button layout."

Having three attack buttons also corresponded to having weak, mid, and strong attacks, which matched their vision of the game being simple to understand, but deep.
Contributed by ProtoSnake