well do the math. Random numbers!
100 damage for each round/life bar. 200 total to win.
in a 3 round system.
round 1 - you do 90 points of damage to opponent, he does 100 to you.
round 2- you do 100 points of damage, he does 90 to you.
round 3- you do 100 for the win, he does whatever.. 90.
total damage done- you did 290, he did 180
In injustice, it's the first to deal 200 points of damage, period. If you dealt 90 for the first life bar, that 90 still stays.