On a more existential level: I'm pretty sure that 3-legged cats (whether congenitally or surgically so) don't bother themselves with such matters. They make the necessary physical and mental adjustments and carry on.
One of my veterinary colleagues had a 3-legged (only one HL) cat and could move just as fast as a 4-legged one; she could also climb trees with no problem (other than resembling a furry nut hatch).
The ones that impress me are the cats and dogs that have only TWO legs. One front and one back, on opposite sides. They move like a cheetah on a pogo stick
The ones with only two front or two rear legs aren't nearly as athletic
