Musk's tweet wasn't to "imply a 350kW" V3, the implication of calling 350kW "a children's toy" would be that V3 is much more than 350kW.
It could be that 350kW is just "V2 for more vehicles at once", but I doubt that. Tesla needs to up their game with Semi, and I doubt the plan is to tell them "connect 8 charging cables every time you stop". And the only way to do that (and fix the current cable overheat problems) is with active cooling. Something Tesla has been filing lots of patents for.
Whether that would help M3? Well........
IMHO, the only thing that's going to drop M3 charge times is an external reserve of coolant provided to the vehicle - not just ambient temperature, but chilled, to near freezing. With unlimited coolant provided at unlimited rates at the lowest acceptable temperature, heat could be removed much faster than with purely onboard limitations. Which might allow for faster charging. This would of course require a port that can accept coolant. There is none at the standard charge port (although the door covers a strangely large area relative to the plug size). Tesla has also patented an underside charging system, but I don't think we've seen any good shots of M3 undersides.
Note all of the conditions that need to be true for V3 to elevate M3 charge rates, though:
* V3 being able to deliver more current per vehicle, not just more vehicles
* V3 delivering cooling (almost essential with the former case, but still...)
* M3 having (standard or future option) or being able to be retrofit with a coolant-accepting charge port
* Unlimited external chilled coolant improving the maximum acceptable charge rate (it should, but still....)
In short? While it's possible, don't bet the farm on it.
I suppose there is one more possibility - that charge rates are just a software limitation, out of an overabundance of caution that could be lifted in the future. Or a bad-faith presumption, that it's a wilfull maiming of the M3 to push MS and MX sales (highly doubtful) which could also be lifted in the future. But again... don't bet on it. They're larger format cells, it's natural that removing heat from them would be slower.