I had some trouble to setup my old AOC 912Vwa VGA monitor on My Raspberry Pi 3.
So I decided to write it down here, and maybe help some folks ;)
I'm using RECALBOX.
First you'll need an adapter - VGA to HDMI. (Sorry, there's no magic for the hardware part)
Then find out the config.txt file, on BOOT partition.
Backup its content.
Then set those configurations and reboot your Rasp.