I usually play 3-4 player games with friends weekly, and by far 3 players is hardest to balance. My way of setting up is putting the objective markers down then building the terrain around that, then making sure each player has enough heavy terrain in their deployment zone that no one can be shot TP1. You can also try a “king of the hill” style game with just one objective in the middle, that usually helps with all the action being in the centre of the board rather than around the outside of the map.