First switch, Can i ask for 15+ LPA to PWC or KPMG as 3 y 8 m exp. Have offer of 12 lpa in hand so How to initiate for asking more ?

If you have a counter offer, negotiation will be easy.

Yes. I know someone of less than 3 yrs exp, who bagged 16 LPA, showing existing offer of 13 LPA. Though it was not with KPMG, but with a Tier2 product based company.