Why should there be? Landgoing animals evolved from fishes. Fishes have fins on all four sides, but when they became amphibious, the fins on their sides allowed them to move on land. The genetic modification that made the side extremities strong enough to move the animal accounts for all animals using side extremities to move, and for symmetry reasons (the number is the same on the left and right side of the body), 3 legs are not an option; or rather, when mutations do happen so that animals are born with 3 healthy and one deformed leg, because of the disturbed symmetry it is a disadvantage.