3 phase heater to Single phase
A long answer, as I get slightly worried when reading such questions. Messing with such powers in installations that are not designed for them can be a quick way to start a fire...

The simple answer is Yes, a 3ph heater at 230/380V will generate the same heat when powered with three 1ph 230V wires, as long as there is a return wire to connect the neutral wire to.

You could run it at 1/3 capacity by simply connecting 1 phase and the return. If you connect all three phases to a single phase, you would probably overload your wiring, as these are mostly rated at something like 3 to 4 kW. Please don't do that ;-)

When powering all three elements from a single phase, you probably need to rewire the return circuit inside the heater. At least check that it has heavy-gauge wire, at least 10mm2. In a proper 3p system, the return currents cancel each other out, but when using 1p they do not. Therefore the return circuit must be heavy enough to handle 50A from the point where the wires from the three heating elements come together. It may be that the heater was manufactured with a thick return circuit, but it may well be not. If the manufacturer counted on the returns cancelling out, the return wire may well be a flimsy after-thought that will burn through in 1p use.

To safely power the whole heater from a single phase, your electrical system would need to be capable of supplying something like 50 A. The cable running to the heater would need to be at least 2x10mm2, or 2x25mm2 for longer distances. That is not cable you can pick up at the home depot... Alternatively, you could have separate cables for each phase. As most home-wiring is rated at 16-20A max, you would need to do that anyway. But then you need to make sure each wire is connected to a different fuse-group.

When using three separate cables, make sure they are the same length and type, to prevent a single return wire from being use more than the others.

But the limiting factor will probably be the rating of your electrical installation: at least where I come from (Netherlands), it is very expensive to be able to draw so much power single-phase. But in e.g. Germany and France, it is more common to have such power available.

Please note that installing such a system is not easy. Proper fuses need to be used that are able to interrupt the high short-circuit current, and the system needs to be grounded with another wire of at least 10mm2. If something goes wrong with the system and the wire touches the enclosure, the ground must be good enough to cause the circuit breaker to trip. If the system can not be grounded, there should be a differential circuit breaker capable of handling 50A.