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

Parrots 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 skills of the natural limbs.