I'm no electrician, although I've done a lot of electrical work, single phase only.
But, since no motor is involved, I'm guessing the reason for three phase is to handle the current for all the heating elements.
If you look at one of the wires, think three times that size if it was single phase.
Same for the circuit breaker, for example, a 3-phase breaker at 60amp per leg would require a 180amp breaker for a single phase circuit.
Not ideal, if it even meets code.
If nobody here can help, I'd suggest going to the PizzaMaking forum.
There are a couple people currently building out pizza restaurants there.
You could ask your question there and maybe they could help (or ask their electricians
)
I know there are posts about the Bakers Pride ovens, but not sure if any involved electrical questions.
EDIT: Got to thinking, why would he want to convert to single phase anyway?
Cost of running heavier wire and bigger breaker would most likely cost more money.