if you do 3 mins straight, your punch output will suffer. No one can go flat out for 3 mins. Just like no one can sprint flat out for 800m (yes u can run fast but it's not flat out). For punchouts, i find the tabata interval protocal excellent. 20 seconds of FLAT OUT punching following by 10 seconds rest. Repeat at least 8 times for a total of 4 mins work The idea is you go FLAT THE **** OUT. No holding back at all.
Yeah, I do 3min, but i usually gas out and just keep going, not full power. I'm starting to go all out when i hear the 20sec. ring.