@Klundell
I guess I disagree with the statement that back ext and sit ups are accessory? You are all right the core does have activation during squats . Your Core ( TFL to be detailed) has to contract before you do anything, it is an involuntary, ie your brain does it for you to stab your self. These accessory movements are the most important! unless you are strong in your erectors , obliques, lower / upper abs, tfl , spinal rotators you really should not be doing large movement exercises like dead lifts. Have to think of it like this , when you do mult mm exercises like squats , dead lifts etc. the strong muscles get strong and the weak stay week. This is the bodies natural way, ie compensation. Picking 3 ex is very difficult because as a skier or any other sports everything should be strong ie, every mm group should be worked. I would say that the most important is the "core" , hip abductors ( glut Med) and hip ext (glut max) and HIP ROT. But like I have stated everything is important! LOL.