Y.K.
honestly i don't know how to put breastfed baby on a 3 hr schedule. i believe in nursing on demand. breast milk dissolves much faster than formula, maybe your baby is going through a growth spurt? or maybe he didn't eat sufficient amount of time at the previous feeding. i noticed when my babies were sleeping through the night, the ate more during the day. i say you're lucky to have all night sleeper, so why bother trying to change that. besides when the baby is crying because he's hungry, what are you going to do, make him wait another hour???
also nursing is very soothing for a baby, so maybe he is going through a tough time during the day or wants to fall asleep and all that requires nursing. sorry i don't have any suggestion, both of my girls nurse every 1,5-2 hrs at this age, in few more weeks the time will stretch to 3-4 hrs.