Which three animals walk on their front leg and hind leg of one side then the front leg and hind leg of the other side?

Camel for one.

Not horse, not cat, rabbits hop, crabs crawl sideways on six legs.

A camel moves both left legs forward, plants them, transfers all weight to left and moves forward at the same time moving both right forward to plant and transfer. A very odd, shifting gait but very secure and firm.

'It was while watching the opening scenes of Disney's "Aladdin" that I realized exactly what made the camel's gait different from the gait of most other animals. A horse or cat walks by moving one front foot forward, after which the opposite hind leg moves. For the next step, the other front foot moves, followed by the opposite hind leg. To observe this for yourself, get down on all fours and walk; you'll notice that if you move your right hand forward, your left knee follows automatically. But a camel walks by moving a hind leg first, then moving the front leg on the same side.'

Giraffe also!!!

All camelids do this (there are more than 3)- camels, llamas, guanacos, alpacas. Horses that are pacers do it. A few dog breeds, such as the Fila Brasileiro, are natural pacers too.