Ebert: Four Legs Good, Three Legs Bad
This is why Roger Ebert is such a great critic. In his (negative) review of War of the Worlds, he writes about Ray Ferrier, the Tom Cruise character, and his battles with three-legged aliens. Then he drops a little evolution-related education on his readers:
If evolution has taught us anything, it is that limbs of living things, from men to dinosaurs to spiders to centipedes, tend to come in numbers divisible by four. Three legs are inherently not stable, as Ray demonstrates when he damages one leg of a giant tripod, and it falls helplessly to the ground.
I think it would be more accurate to say "divisble by two," since insects have six and most centipedes have 70. But the larger point is that even fantasy and sci-fi movies are better if we find the creatures they produce plausible, and three-legged beings won't feel right to us, even if we can't articulate why.
NOTE: For a good example of what we as Darwinists are up against, read this.