Hi
You say your coils aren't connected yet, if it was designed to give 24V in star could you possibly connect them in delta, that would give closer to the 12V you are looking for.
A 3KW inverter at 12V though is a lot of amps if ever you load it to its full capacity, 250+ amps, that is going to be cruel on the batteries. If you are doing it just as a hobby for fun OK, but if you need the power I would seriously look at a 24V inverter.
Brian