there are no critical hits for your guys in chapter3 (it starts in chapter4).
make sure that you have valhalla serum. in case that you are using burge->britt each round (the order is important), and still miss some damage, if you chose to take fitness in borge's mission to train the boys you can try to redo the chapter and choose the endurance (will) choice. it usually gives you 1 more round.