This is a combat concept.
(Skillfully avoiding the obvious jokes) nobody has 3 legs like a stool or a tripod.
If you wobble a tripod it's balance will redistribute to the 3rd leg.
Wobble a person and they have to either step to where the third leg isn't or they fall over.
This is Third Point theory - the place where there is no leg.
Throw your opponent to the third point without allowing him to step to regain his balance and he will fall that way.