There are a total of four legs (two people each with two legs). Since two of the legs are tied together, it restricts mobility as if they had only one leg. Therefore, you have two indepedent legs and one functional tied leg, making it a three-legged race.
Three legged stands are often called stools. In photography a three legged stand is called a tripod.
One eyed three legged donkey
a tripod
a tripod
A teapoy
your moms is a one legged woman with three arms and they call her one legged timmy
lame or disabled, personally I would call it useless and dogmeat. NO!!! you call it a horse missing a leg smart ass!
Photographers normally use a tripod (three-legged stand) to hold a camera steady.
A tripod
random
two people singing together is a duet
decapod