Replace door mirrors on EA Falcon wagon

My wife has managed to destroy both door mirrors on my EA wagon. I have replacement mirrors (manual-not electric) but I can't figure out how to access the third screw so that I can remove the old one.

I have removed the mirror trim from the door, and removed the first 2 locating screws. I couldn't find the third screw so I removed the door trim to get a better look but that didn't help me at all.

Can you give me some kind of guide as to how to do this?