if it is a 12 wire head, which i suspect it is because it is built for the more common world market
simply connect it as wye, then take 120 off of each phase referenced to the neutral position.
now you will be limited to 2/3 capacity of the head, in most cases unless you are feeding a 3 phase panel and want
3 circuits of 120 single phase then you can get full capacity. if you want 220/240 that you can't have, although any two legs of
a wye connected head will provide 208 vac which might still work out for you. depends on what you need or want and what the future
goals are. if you have no need for 3 phase your probably better off selling or trading your head and get a single phase unit.
(if i am wrong on this someone will jump in and say so i am sure)
failing that, how big of a head is it? i got a few single phase heads of various sizes i would be willing to trade
you out of your 3 phase head with.
bob g