where to add collison file
You can either add your collision file into one of the col files that are located inside the gta3.img file in /models/, Extract the airport.col onto your desktop add your mods col to the airport.col
Delete the airport.col in the gta3.img before you place your edited version back into the gta3.img and then remember to rebuild the img file after.
Or you can add your new col into a folder inside your models folder. Create a folder called Yourmods inside your /models/ folder.
open your gta_vc.dat file found in /data folder and add the lines below.
COLFILE 0 MODELS\COLL\GENERIC.COL
COLFILE 0 MODELS\Yourmods\Yourcollsion.COL with your col models inside.