In most cases, two forwards forecheck and the third forward has to have a lot of discipline with their positioning to make sure that all of the time they are looking for an offensive opportunity they are also ready to bail on the offensive zone and get back to defend.
Same thing with pinching. If the forward on the side of the pinch is free enough to cover for the defenseman pinching he does so, but the forward on the other side of the ice has to recognize when that isn't going to happen and back off to provide defensive support.
It's all about discipline, which comes down to coaching which is why the Rangers suck at it. Quinn seems to focus on the forecheckers having "good intentions" which leads him to value guys like Howden who look like they are putting in a lot of effort on the boards, over players actually being good at positioning where you are already in the right place and don't need to look like you are trying so hard (for example, the Isles have gotten so good at their system they don't look like they are killing themselves to maintain that structure).