It depends on the time of day, and the demand.
During Magic On Demand, they may share... they may not... it's all dependant on demand. For example, I was driving out of the All-Stars today, and due to the rain, it was very quiet. As such, we were doing 2 or 3 of the resorts... During busy mornings, each All Star will get thier own bus.
During afternoon hours, Studios, DAK, and Epcot share. During Epcot's close, they will split them. MK always has them split.
At any rate... even when they share, the resorts are very close, and it doesnt take very long at all to go through all three. Bus service to the All Stars is typically better than most resorts, because of the sheer number of people staying at the All-Stars. Busses will typically run every 5-15 minutes, on average.