Mark,
The start cart is the unit that provides start air to all engines. The cart is plugged into the aircraft and provides low pressure (high volume) start air. The high pressure (bottle) is only able to start # 3 engine - and you get only one shot.

On the overhead panel you select the start air you want with the switch in the middle of the start panel.

I'm on my way out the door, but I'll look for the procedure in the manual later.

Lou