Tripedalism

Tripedalism is locomotion by the use of three limbs. Real-world tripedalism is rare, in contrast to the common bipedalism of two-legged animals and quadrupedalism of four-legged animals. Bilateral symmetry seems to have become entrenched very early in evolution, appearing even before appendages like legs, fins or flippers had evolved.

In nature

s are the only creatures to naturally use tripedal forms of locomotion, as they use their heads as a third limb when climbing. They generate propulsive and tangential forces equal to or greater than those of forelimbs in non-human primates when climbing vertical surfaces.
Non-standard tripedal gaits are also observed in primates when they use one limb to grasp an object.

Quadrupedal amputees and mutations

There are some three-legged creatures in the world today, namely four-legged animals which have had one limb amputated. Animals made into tripeds through amputation may suffer from long term health consequences; these issues include osteoarthritis and spinal deformity and are largely a result of disruption to natural locomotion cycles and symmetry.
There are also cases of mutations or birth abnormalities in animals which have resulted in three legs. With humans, a third limb may arise from the surgical bisection of conjoined or parasitic twins. Such conditions will often result in a limb with reduced or limited mobility. In rarer cases such as with Frank Lentini, a third limb may be usable in activities requiring moderate motor function, such as kicking a football, but will still lack the fine [motor skill]s of the natural limbs.