You'll find a lot on line but (an oversimplification), if you have 3 attackers, then they are outnumbered by a defensive 4: LB-LCB-RCB-RB. So, you need to overload on one side by the wide attacker going in between the back and centre back – the half space (inside forward as it used to be) – and another player going outside, usually the full back/wing back, with a midfielder pressing up; these positions may be rotated. Generally the wide attacker is in the best shooting position on his 'wrong' foot. I hope that makes sense, just think Liverpool / Man City and the way they do it.