I think thats so you can be in the cab and still look outside, they I believe are called "windows" :P
just kidding. the three screw holes on the drives door are for the factory mirrors. I have a brand new set of stainless rectangular ones from LMC that I will be using. my passenger side doesnt have the mounting holes though, I will have to drill.
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=47
I wish they were the round ones but they came free with a parts truck so beggars cant be choosers.