For defending: keep your players in a primary secondary format. This meaning one player goes to be the defender on ball, yelling "got ball". The secondary player gets into a support position, about 2-3 feet diagnal from the primary defender. This prevents splitting of the defenders as well as if then primary defender gets beat, secondary is there to back him up. The third player should be in a offensive position, waiting till the poossesion is changed for a quick counter.
For Offense: keep everyone in a triangle, have loud communication, take on players in 1v1 situations, and have 2 players go to goal, while one hangs back just a tad