Power and control 3 phase ac induction motors with dc source
What you want to do is entirely possible, but what is your timeframe and budget? You might be better off taking an off the shelf DC-DC converter to get your 48V up to ~500VDC and then feeding that into the DC link of some off the shelf variable frequency drives.

Variable frequency drives take 3-phase AC, rectify it to a high DC voltage and then pulse-width modulate six IGBTs to chop that DC bus into three-phase AC of any voltage and frequency to give you easy control of AC induction motors. Almost all of them allow a "common DC bus" configuration mode where the rectification stage is unused, and this is what you'd want to do.

In fact, even if you have to design your own 48V or 96V to 500V DC-DC converter you'd be better off than trying to create your own three phase PWM drive. I'd start looking at the DIY electric car enthusiasts for tips on this DC-DC converter.