How to power several gate drivers with as few power supplies as possible?
Upon NPC configuration  you may save 3 DC-DC.

If your layout is TNPC i.e. your neutral switches ( horizontal branches) are made by two anti-series IGBT with their emitters joined then each couple can share one single DC-DC (and probably gate drivet too).

Other possible topologies unfortunately cannot.

Being connected to negative bus-bar you may also be tempted to use one single DC-DC for your low side switches. This should be really avoided though, the emitters stray inductance would otherwise l kill your di/dt