It really depends on the season, crowds, and time of day whether the All-Stars share or not.
During the morning, typically each All Star gets its own bus dispatched to the parks... However, if things slow down a bit, a bus might do two or all three. It depends on the demand.
after 11am, Studios, DAK, and Epcot will run all 3 All-Stars. Busses are dispatched to alternate between which side to start on... so, sometimes Movies will be first.
After 9pm, Epcot typically splits up the All Stars for the Illuminations rush.
MK splits them 98% of the time.