I appreciate this is almost certainly not a GSX issue per say.
Of course it's not.
How can I get it that door 1 opens passenger door 1 and 2; leaving door 3 to just open the catering doors 1 and 2.
This gets confusing when GSX is configured to use 2 passenger doors but only 1 is opened by the correct door 1 keys.
It's important you keep in mind that's NOTHING about how doors
operates on a specific airplane will ever be touched by GSX or is affected/changed by GSX.
GSX only need to KNOW how doors on any specific airplane are linked to FSX variables, so it can check for their status and allow services to continue or not.
From GSX side, you can only:
1) Configure a door exactly as it is in FSX, this is the most correct choice
OR
2) Configure GSX to IGNORE a door's status, to allow services to continue without waiting
OR
3) REMOVE a door, so GSX won't even use a service there
If you want to CHANGE how the door
operates there's nothing you can do with GSX, you must act on the airplane configuration, which is controlled by BOTH the aircraft.cfg file, which is easy to do, but ALSO from the airplane .MDL, which is not very easy to do.
The aircraft.cfg file controls the relationship between the Key assignment and the internal FSX variable. In the [Exits] section, the 5th parameter after the x,y,z coordinates is what tells FSX how many doors this airplane has, and which Shift+E+XX key controls them.
For example, if the 5th parameter is = 2, you are telling FSX that when you press Shift+E+2, the FSX internal variable that represent the doors status, which is EXIT OPEN:2, will change its value.
However, changing JUST the aircraft.cfg, will be ok for GSX (since GSX checks those variables) but it won't really "open" the door, visually. Yes, the internal variable will change, but the actual animation is stored in the airplane .MDL file, so you must change that one too, in order to SEE the door opening.
That's not easy to do without doing a bit of hacking of the .MDL file, like editing it with an hex editor.
Only AFTER your aircraft.cfg and .MDL file are in agreement and the airplane
operates as you want, you can configure GSX to reflect its new status.