hmm, I think they are all the same. 'mempunyai' comes from a root word 'punya'. i mempunyai, simply u add som affixes 'me' and 'i' the formula will be me+punya+i it'll be pronounced as mempunyai. For me, 'punya' sounds less formal than 'mempunyai' :D
I hope it helps.