Most of the airports in G3 come with multiple airplane spawn locations. To change which airplane spawn is used, you must change the camera position. Look for "Camera Position" in the "View" menu. Clicking that brings up a submenu of all the pilot spawns in that airport. Once you choose a pilot spawn, the plane is placed at the closest plane spawn. Press 'X' to cycle through the list of pilot spawns.

If you want to add your own airplane spawns or change existing ones, you may do so in the airport editor. Click "Edit _______ (airport's name)" in the Simulation menu. Once in the editor, look in the "Airport Editor" window for the various plane/pilot spawns. They are usually in a folder called "Control Objects." You can move/delete/rename them just as with any other airport object.