Im pretty sure under the title line all you need would be [Lets say fltsim.0] 'Thai Airways International Airbus A300-600 GE' and so forth. The AC#1 only relates to your flightplans.txt and aircraft.txt folders so it knows which aircraft to use, and the title in the aircraft.cfg relates to the 3rd part of your aircraft.txt, and finally, 200 represents the maximum cruise speed (dont quote me on that). So in the end what you would end up with is:
AIRCRAFT.TXT (To be compiled with TTools)
AC#1,200,"Thai Airways International Airbus A300-600 GE"
AC#2,200,"Thai Airways International Airbus A300-600 PW NC"
AC#3,200,"Thai Airways International Airbus A300-600 PW OC"
AIRCRAFT.CFG (Root aircraft folder)
[fltsim.0]
title=Thai Airways International Airbus A300-600 GE
sim=A300
model=
panel=
sound=
texture=Thai Airways
....
....
[fltsim.1]
title=Thai Airways International Airbus A300-600 PW NC
sim=A300
model=
panel=
sound=
texture=Thai Airways
...
...
[fltsim.2]
title=Thai Airways International Airbus A300-600 PW OC
sim=A300
model=
panel=
sound=
texture=Thai Airways
...
...
Hopefully that explains it
Please direct your complaints to the moon, they might care there.