The main advantage is that the 'footprint' of a three-phase transformer is smaller than a transformer bank, and all connections are prewired which makes them easier and quicker to install and replace.
Three phase power uses 3 conductors (or sometimes 4) for the equivalent power of 3 circuits, that in single phase would require 6 conductors. This is possible because the voltages and current are 120...
A three-phase motor will not start if one or two phases aren't connected. If while running one or two phases "drop out", it will continue to run for awhile, but will eventually burn out, unless...