You 3 man until after you beat the boss and then offer to make a freeloader's day.
You can actually 2 man everything except for the absolutely last part. The 2 pillar gate can be done with only 2 players by having the players hug the force wall before clicking (you still need to be at least 10m away from the pillar you're clicking, so both players will be hugging the central third of the force wall). When the wall comes down, one of the clickers simply moves through; the wall will be back up, but you're already on the other side of it. Repeat for the second person.
Supposedly, someone has 3 manned the final part of the mission, which, in every attempt I've ever made, has required 4 people. You can *reach* the side clicks and the final click simultaneously from the proper vantage point on the sides, but, as soon as you stop the side click, you're incapable of completing the final click. Unless there is simply some kind of wonky geometry screwery that lets you get past one of the clicking requirements without actually having to click, you're not going to be able to 3 man it.