How do you step up 12vac 3phase to 24vac 3phase?

Twelve volts is not a common three phase voltage. Assuming that this is a hypothetical question any 1:2 ratio transformer will do the job.

Rather common for electric fork lifts

very small ones use 12v (hand stackers,pallet jacks)

larger ones are 24v 36v or 48v

the original Transformers on the chargers usually run from 480V

and the chargers are specifically designed for the battery

most chargers use half wave rectifiers in a star configuration

600A thru a .7v drop for a diode is 500w loss, full wave is 1000w

regulation is switched from the primary side since its easier to handle the low current