All these VFD's will be rectifying the AC input to DC, and then doing stuff with that to make the 3 phase O/P. For low power (and low cost) you can use a capacitor/diode voltage doubler on the input to generate ~650V DC, alternatively for larger power you can use a switchmode Power factor Corrector (PFC) stage to do the same and also keep you legal for harmonic currents. This will need just one (or a few) small decent inductors, if the switching frequency is high it's quite possible to get 3 KW through a couple of cores maybe 35 or 40mm across, larger powers pro-rata, so you don't need massive transformers.