What is the Best Way to Reduce High Power Three Phase Rectifier Ripple?
Capacitors are the simple way to do it, but you can decrease the amount of capacitance needed by putting a DC link choke in the circuit after the rectifier. A choke would not do it all, you still need caps, but you can make a choke cheaper than you can buy caps. 

With the caps however, you must be careful about inrush current, so generally that is dealt with by using a "pre-charge" circuit of either a current limiting resistor that is shorted with a relay / contactor in line with the DC circuit ahead of the caps, or using an NTC thermistor (up to a certain power rating).

The multi-phase transformer method requires more diode bridges and is generally used to reduce the harmonics created on the line side, not the DC bus ripple.