How do you Connect 3 phase water heater to single phase?

Most 3-phase welders cannot be made to function on single-phase power. If your welder is an inverter type, such as a Miller Dynasty or Lincoln Invertec, then it may have a single-phase mode with reduced current output. You would have to download the manual for your particular welder. If it is an older transformer (big and very heavy), then you're probably out of options here.