I'm always gassed out by the 3rd (3-minute) round while shadowboxing but I think it's because I go all out on my shadow. But regardless, how do I build endurance and stamina to go the distance incase I don't knock my shadow out?
If you are going all out than just keep doing what you are doing. Endurance is built through time and persistence. Learn to relax and to breathe.
This 100%, pushing yourself in training till you are gassed is a good sign because that's what's necessary for u to have the adaptations to have better cardio. Your body will get stronger and build the necessary muscles to last you longer, and subconsciously you will work harder to learn to throw more efficiently while keeping the same power and intensity Some things you can double check to make sure other than breathing and being relaxed is to pace yourself and to improve the efficiency of shots by constantly trying to improve the mechanical efficiency of your shots (while maintaining defensive responsibility of course) like rotating on your shots, tight core, driving up from the ground, etc. Just a side note. If you are worried about this for the implications it brings to possible fights in the future or sparring, try and pace yourself during those things and practice it, but being gassed out in training is never a bad thing, because thats what makes your gas tank stronger (as long as you are eating well and recovering properly of course)
I missed the last part where u asked how u can make sure u last till the end of the third round Look to train sometimes to deliberately pace yourself for three rounds, set a timer that are 3 (3 minute rounds with 1 minute rest) and train according to that timer, this will prep your body and mind for those rounds and will encourage you to pace yourself. Pacing will be everything, in order to have energy in the third with your current output, youll have to take a little away from your first and or second. But when you get better (by pushing yourself and gassing yourself out in training) youll be able to have a larger overall capacity for not getting gassed out. When Im not sure, I just go lighter in the earlier rounds and go all out in the last round