Campers often use a cooking stove with three legs. which postulate explains why they might prefer this design to a stove that has four legs

Explanation:

The very fact that there exists always a plane passing through any set of three noncolinear points assures you that you can place the cooking stove with three legs.

Of course, if you use less than three legs (e.g. two legs) the cooking stove would not be stable.

If you use a cooking stove with more than three legs, for isntance four, it will be difficult to find a plane spot that coincides with the plane  formed by the tips of the cooking stove, such that the cooking stove stands leveled and in stable equilibrium.