The backup crew and some support crew participated in an interesting rescue scenario with 2 astronauts flying to Skylab to take back the crew of 3 onboard Skylab. This would mean 5 astronauts returning to earth in the command module.
Neutral Buoyancy Simulator at MSFC
Gibson Training at ATM Simulator
3rd Crew at Mission Training and Simulation Facility at JSC
Aside from physically training for the longest crewed missions that the United States had ever attempted, the actual training program was developed simultaneously with the hardware and experiments for the mission.