Possible methods to control the voltage of a variable 3 phase power supply for an electromagnet?
This answer was made before the OP mentioned that the target load is an electromagnet. I suspect that if the electromagnet is purely dc (i.e. uses a 3ph rectifier) then a dc solution will be simpler.

Original answer: -

If you considered a 3ph motor controller inverter you would get a variable output frequency and voltage: -



This method is clearly tailored for a motor but there is no reason why the main power electronic components couldn't be modified to produce a 3 phase, variable frequency and variable voltage power supply.