[p-squared]

> Three points define a plane; it's guaranteed not to wobble.

This gets repeated a lot. I think if you stop to consider the common case of non-planar ground, you will see that this mathematical factoid isn't actually super-relevant.

(I will concede that it's not trivial to add a fourth leg and make it co-planar to the other three. But this often isn't the cause of wobble.)

[colanderman]

It's more relevant that three legs have three degrees of freedom (ignoring rotating or translating the table), and thus can each sit at an independently chosen height.

(It is conceptually satisfying that once you throw rotation in, the added degree of freedom permits four legs to meet the ground!)