Destiny: The Taken King Guide On How To Use Three Of Coins
Destiny: The Taken King's Three of Coins is a new consumable item that, according to Bungie, “gives bosses a chance to drop Exotic Engrams, including Weapon Engrams. These consumables have a cumulative luck effect, so even if one doesn't net you a drop, the next one has a higher chance to do so.”
This shows you should only be using one coin per Ultra you kill. Based on the description from the weekly update, using more than one Three of Coin is in fact a complete waste.
Farming Scourge of Winter story boss, for example, get the checkpoint outside his chamber. Pop coin, snipe boss low, run in and rocket suicide to kill him. If he dropped loot, it'll still be there.
If you prefer watching at a video to fully understand how to use Three of Coins, take a look at the clip below, it's really clear.
If you need any help then let us know in the comment section below.