I have one 30A 120V single phase circuit and three 30A 120V single phase loads. How do I switch between the three, i.e one is on and the other two are off?
The 3 position switch Johnny pointed out is listed to UL 508 for industrial control devices, so it should be acceptable for switching some general-use receptacles around. (You'd be creating a tiny industrial control panel under Article 409 of the NEC -- your average junction box + cover combo meets NEMA 1, and there are provisions in the Code for an industrial control panel that's protected by the upstream breaker.)
Can they be prioritized? If so, magnetic motor starters could be used, such that the highest-priority load drops the other two starters out of circuit, and the middle-priority load drops the least-priority load out of circuit. BUT... momentarily, all three COULD be energized at once unless you used break-before-make starters and drove each actual load from its own motor starter.