If we're talking historical situations, Manchester's well-known "circular" service 53 (Cheetham Hill to Old Trafford) used to run very frequently in MCT/Selnec/GMT days and was worked by buses from 4 garages - Queen's Rd, Hyde Rd, Birchfields Rd and Princess Road.
Prior to deregulation, and especially before the PTEs, there were quite a few "joint" services in the north-west worked all-day and every day by not just multiple garages, but multiple operators.
Triple-operator examples were:
- Bolton/Manchester service 8 - operated by Lancashire United, Bolton Corporation and Salford Corporation vehicles.
- Liverpool/Wigan service 320 - operated by Lancashire United, Ribble and Wigan Corporation buses.
- Bury/Manchester service 35 - operated by Salford, Manchester and Bury Corporation vehicles.