I've found my best success with 3OC to not set out to use them but simply play the game and use them when the opportunity arises. Messing around on the dreadnought today in the span of about an hour and a half I used probably 4 during C.O.O tier 3s (no success there), 3 during the Hive/Cabal champion fights in the Hull Breach and got an exotic helm from one and a gauntlet from another, and used one while doing the agnorach (sp) rune on the zealot and got another exotic helm. I'd say I average roughly one engram per 5-6 3 of coins, but I will say that at least 60% - 70% of the drops are armor, not weapons. Don't know if this is the case with everyone else or not.