My pediatrician told me to nurse my baby every three hours. Does that mean from the start of a feeding or the end of one feeding to the next? Because his sessions sometimes last an hour.

When I was breastfeeding, I was told to feed on demand instead of waiting for 3 hours after each feed. This is because we're not able to see how much the baby is drinking and babies afterall are smart enough to cry each time they're hungry. Breastmilk digest a lot faster than formula milk so it's no surprise if they get hungrier faster. But as they grow older, like Reine mentioned, the frequency between each feed gets longer too.

Count from the start of the feeding as they will start digesting the moment you feed them. When baby is younger, most of the time will be nursing but it will gradually get better.