Is there any animal that can walk, swim and fly unaided?

Puffin. Like most birds they can walk and fly. In addition their short wings are adapted for swimming with a flying technique which means that they can swim well under water.

Any diving duck

In fact, any duck, goose or swan, any gull or tern, phalarope, currently available auk (including guillemots and puffins, but not the flightless Great Auk - now extinct), and probably skuas too. Cormorants and shags as well. Divers (loons if you're American) are a bit dubious - they fly and swim, but to describe their land motion as walking is stretching things a bit. Not sure about grebes, but as they're built somewhat like divers (legs very far back under the body), they probably shuffle on their tums like their relations. Source: Field Guide to the Birds of Britain and Europe, by Peterson, Mountfort and Hollom (Collins).