How can I switch between three devices on one 50A circuit?
If you are trying to connect switches to three different outlets the easiest method I can think of is mount a sub panel closest to the receptacles and connect each receptacle to a different 50A breaker. I think trying to find and purchase a heavy duty 240V/50A 3 position switch would probably be a little out of a DIY price range, as it is an industrial piece of equipment.

Hope this helps

FYI - From a commercial standpoint you can connect 50A receptacles together on a single circuit. I have done but the AHJ made us label each receptacle that only one receptacle could be used at one time.