HI DL mech, Buzz here. Sir VC10 is right, whoever ordered that one wanted one more autopilot aboard. All our Classic Lumps (-100's) are retired, we have some ex-Qantas (-238's) and they're 2 autopilot ships.
The triple autopilot is Boeing's way of ensuring Cat 3b landings. The 737-300's are a Cat 3a animal, with 2 autopilots.
The A320's we have are only a "2 autopilot" affair, but each is a 2 headed monster : command, and monitor. So 4 channels have a death grip on the ILS in a coupled approach.
THe difference between Cat3a and Cat3b? Well, Cat3a (737 stuff) has a 50 foot ceiling (half a wingspan up) and 700 foot visibility (you can see 4 seconds ahead of you). The 737 doesn't use rudder for autopilot work.
Cat3b has zero foot ceiling (can't see your shoes due to fog) and 400 foot (OK, i forget exactly) visibilty. Figure that you land about 120 mph..... 176 feet per second...... the airplane has to keep you straight on the runway.
Ever wonder why we mechanics go to school a lot? When we make mistakes the press thinks it's a big deal or something! (grin)
Buzz Fuselsausage Line Mechanic by night, DC-3 Crew Chief by choice