I'm afraid it's just a matter of time! At 12 weeks many, many babies don't have big enough stomachs to manage the full 3 hours, or any less than two night wakings. Both for your milk supply and your baby's well being it is so important to make a routine around what best meets their needs, and if that's 2.5 hrs at the moment, then so be it. Keeping the EASY order of things, but adjusting it to your baby's time scale. It may change and lengthen next week, maybe next month, but be confident it will get there.