They are Pomacea bridgesii, Pomacea Haustrum, or Pomacea canaliculata. Bridgesii are cool because you can have them in planted tanks. The other two are plant eaters.
What atrracts people to these snails is the variety in colour and just how interesting they are. What amazes me about them is how they lay their eggs. Look at how they lay them in the wild: http://www.flickr.com/photos/geekmom/1021523743/
They are just so interesting and fairly easy to breed.
I would suggest buying more than three, just because then you can see them interract with each other
As i said, some eat plants, but the brigs do not. They also need water rich with calcium for healthy shells. Most snail keepers will dose their snail tanks with liquid calcium *like the kind used for SW