rear arm rest screw size 65 gto
 

what the size of screws holding rear arm rests 65 gto are there 3

They're used across the board 64-67 and if you have any you'll realize it just by the size of the larger screws.
If you don't have them AMES does.
Hope these pics help a little.
Cheers

FYI, if you have a Fastenal (or similar industrial HW supplier) they sell these (and all kinds of fancy screws, nuts, bolts and stuff with every kind of head and material option you can imagine) by the box for a fraction of what the restoration suppliers sell them for, but you will need to know the technical size (#), or bring a couple with you to compare.