I'm going with the "combat" tag for this answer.
Backstab is not just attacking from behind (or from a vantage point). It's waiting for the exact moment for the strike that should kill. It's the melee version of sniping.
This means the victim should not be aware of the assassin (for this is, semantically, an assassination attempt).
The moment you know some clown is playing around trying to get you, your behaviour will change.
You know that, in the melee, there are three opponents trying to get you and your group, and you move accordingly, trying to not get yourself in a stupid position). The moment you know there is a fourth opponent trying to play "ninja", your behaviour which change accordingly (moving in random ways, never losing the "ninja" from sight, and if lost, then moving against a wall, and increasing randomness of moves, etc.).
My guess is that once someone knows there are backstabbing in the melee, then there's not way to try one.
Now, if the victim believes there are only three opponents and no surprises, his/her behaviour will get more confident, taking only the three opponents into account, and open him/herself to a backstab attempt from the fourth ninja.
Now, your rules/mileage may vary.