Opening Doors, and Moving Last

[ColBosch]

You're all missing a crucial fact here: you cannot simply declare you're waiting to see what everyone else does. The entire point of the initiative system is that someone is going to be forced to move first. If fighter C wants to join in the combat, they have to roll initiative like anyone else. If they happen to roll better than B, then sure, they can move into their rear hex.If they roll poorly, they can choose to not move at all, but that means they could end up sitting out the entire fight waiting for "just the right moment."

Bursting though doors, guns blazing, is such an action trope that as long as the situation doesn't preclude it - the door is magically locked, etc. - I'd allow the "action" to be just a small penalty to movement. Even if a different GM ruled otherwise, two against one is deadly odds in TFT. No matter how fighter B pivots, A or C will be able to get into their rear hex and attack. The difference between "screwed on Turn 1" and "screwed on Turn 2" is mostly academic.