Unless you have a huge UPS or a back-up generator, how would switching between supplies help you?
If a navvy puts a pick into the power cable in the street, all three phases will go down!
An intermittent supply will affect all three phases.
An over or under voltage supply will affect all three phases.
A frequency variation will affect all three phases.
If you overload the supply fuse, switching the overload to another supply will trip/blow that fuse too.
I can't see any advantage in being able to switch supplies unless, as I said above, you've a back-up supply of some sort that is independent of the mains.