Does using more than one Three of Coins at a time, (i.e. if I consume 5 three of coins before killing a major), is the chance better of that major dropping an exotic? Or is it a 1:1 ratio of ToC to Majors?
Majors don't mean shit.
Use one at a time. It gets used when killing an ultra (and only an ultra). Your chances increase if you don't get an exotic engram and keep increasing until you kill an ultra.
[spoiler]I'm a little drunk, but I'm 100% sure about this.[/spoiler]
Edit: Ultras are bosses that have a skull by their name. ONLY USE ONE PER ULTRA!
No, only use one at a time. Supposedly if you don't receive an engram from the ultra you kill, the next 3oC will have a greater rate. But I've seen a lot of threads saying that the drop rate is broken has hell.