What kind of electrical savings can be expected between a 3 phase convection oven and a single phase?

Virtually nothing. The only real benefit you would get is if you had three phase vs. single phase fans, and I'm fairly certain even the three phase convection ovens use single phase fans. The oven will be rated at a certain kW usage, which applies whether it's three phase or single phase (...and will be ~ the same).

If the three phase convection oven kW usage is slightly lower, it is because it is heating slightly less (so its' kW rating is actually less than the single phase), and vice versa.

Insulating factors will have a significantly higher impact on cost savings compared to single vs. three phase in this instance.