That involves expensive external hardware converting AC to DC. I'm talking about native 3 phase charging embedded into every vehicle and the charging port design and specification itself.

There are certainly fleets that need rapid charging throughout the day and they will need expensive DC units. There are many fleets that only need lower speed, longer duration overnight charging at the fleet yard. Native 3 phase with load sharing charging hardware is the lowest cost way to achieve that. No need for expensive labour to move vehicles or power connections overnight, much lower installation cost than even the smallest DC charging hardware, and no need for expensive 400+V DC certified electricians.