I think a misconception comes out from these questions. In my experience mature piranhas do not fight because they are hungry, they fight over territory. Over feeding might help because then they are slugish and need to digest. A mature fish can easily go a month without eating so 3 days is nothing.
To answer the question, I think you would be sitting there staring at nothing. The fish would either huddle together or go to opposit ends of the tank. My guess is that it would be at least a day or 2 before anything happened. These fish are not like some people think and they do not "fight" simply for your entertainment.