I have one but only two batteries. I've been charging both packs simultaneously using the P3S charger without any issues other than the charger tends to produce more heat. The only thing I would change is to charge in a well-ventilated, if not airconditioned, environment. Other than that, it works quite well. I think the P3P charger (100W) will be slightly better for this setup... better operational headroom as well as slightly faster charging.
For the 3rd-party car charger that I own, it can't charge more than one battery at a time using the 3-way charging board. I think it doesn't provide enough juice to charge two or more batteries simultaneously. What I did was to purchase a 12V to 220V car inverter and use the DJI charger to charge two batteries simultaneously.