I gather it's crucial too look at 3p AC controller project from the end-user application (kW/Nm needed at the wheels/tranny) in the first place, that's how the pros do it too, and not from the primary perspective what general 3p inverter solutions and hacks can be possibly done..
And that means in our diy case start with the available industrial motors, they come in many efficiency/rpm/pole/weight configurations. Most likely you have to forgot heavy cast iron type of motors, yes they are cheaper and could be sourced from junkyards free. But hey, the AC approach
IS about maximum efficiency, so left this idea of uber cheap motor for the DC controller domain, don't ever forget that.
Great, so we are left with aluminium case option, several US/EU/Asian companies can supply that. What's next? You have to decide on overall drivetrain selection (gearbox or directdrive), 2pole motors will be more suitable for the gearbox and 4pole motors for the direct drive version, but you can use 4pole and gearbox for higher highway speeds capability if that's what you are after.
What is the target peak power & torque (kW/Nm), usually 3p inverters can handle 150-200% overload for dozens of secs repeatedly, that should be enough for average acceleration demands with water cooling (or good air-heatsink). So, for small-medium econobox vehicle application, lets take 50kW as the peak power demand, calculating backwards that's ~33kW nominal spec (for 150%) of your inverter. Btw. the inverter should be at least 400VAC nominal with ~800VDC max input (don't worry the voltage can be split into packs). There is little/no benefit running <200VAC drivetrain systems (for cars), which then need high amperage wiring and large capacity batteries, that's easier/more optimal to return into DC traction system. Even the Prius hybrid's controller (for "assist" emotor) is internally wired for ~500VDC. As compromise, lower voltage solution, you are probably all familiar with the Etischer's project of hacking/upgrading small industrial VFD for higher output (IGBT, caps, software upgrage), I gather this would be the easiest/preferred diy approach anyway (with Pb batteries):
http://etischer.com/awdev/
Now, calculate it backwards for the 3p industrial motors driven (overclocked) on higher frequency/rpm, so you end up with suitable range of 7.5-[11]-15kW (nominal) 4pole industrial motors, the special order models with prepared wiring and termistors for this setup (best brands like ABB) can weight even bellow 65kg.
To refine it and for selection of appropriate motor reffer to those several AC industrial conversions done successfully in australia, they have got quite extensive simulation tables and practical recommendations on their forum.