I really don't understand

I've got 3 stars even with mk5 shooter on several occasions by doing 5000 damage. There's no fixed amount of damage you have to do get 3 stars. You just have to do the damage well higher than average, most of the time you'll get 3 stars. And as you can see my damage is way higher than the average on the battle result, but the game valued the assist of the guy with frost blaster more, so he rewarded with 3 stars.