You could do it with three legs, yes.
But consider, this is a 2D game.
Therefore you'd need to have 2 legs on one side and one on the other.
Now let's say we're talking about a tall tripod, like the Half-Life strider (because, honestly, there's no point make "Crab" sized tripods because you wouldn't notice.
You might have the stroder walking around on a hill lasering things or something, and there's this one clone in front of you that you want to kill.
Code:
O-
-----// \-------C-----
But then this skeleton comes up behind you with a flak cannon.
Now you have two options, you leave the skeleton alone and kill the clone.
Code:
O-
--S---// \-------X-----
Then you are blown up by the exploding flak
OR, you turn around to kill the skeleton.
Code:
-O
--S---/ \\-------C-----
Then it shoots you in the front leg.
Oh dear.
Code:
-O
--S-----\\------C-----
You can't balance on those two legs there, so you fall over and become useless.
Then the skeleton blows you up.