The best option is the Black Garden mission on Mars. You can do it on easy too. Fight your way to the final 3 bosses (The statues) Pop 1 3oC for the 1st boss and kill him. If no engram drops, pop another 3oC and kill the 2nd boss. If still no engram after the 2nd boss, return to orbit and re-load the mission (It will bring you back into the 1st boss again) Once you get your engram, return to orbit, change character and go again. It's my go to place and I have literally every drop-able exotic now - I still do them for shards and infusion gear.

Here's the best way to use them. Go to the scourge of winter quest on venus. Pop a three of coins. When you get to the boss fight, Draksis, equip a rocket launcher (Gjallerhorn is best if you have it). Take his health down to next to nothing and move up next to him. Fire off your rocket killing him [i]and[/i] you at the same time. This will result in a boss kill (potentially dropping an engram) and you respawn back outside his throne room to rinse and repeat. This way all you need to do is pop your three of coins outside the room, take his health down and kill both you and him at [i][b]the same time[/b][/i]. Don't bother stacking them (using 2 or more 3oC at the same time). It doesn't work. This way you will literally go through your coins in no time and it's the quickest way of getting exotics. Just make sure you have heavy ammo synth incase you run out of rockets. And [b]make sure you take his health down far enough that 1 rocket will kill him [/b](hence Gjallerhorn helps with Wolf pack rounds).

The most effective way to use them is in strike playlists. Just pop one for each strike. Stay in the playlist so you take advantage of the bonus.

Not an expert, but I'll try, someone correct me if I'm wrong. You can use a 3oC before any yellow boss (like the ones in strikes) except the ones with a shield (like the psion flayers or Omnigul), and before any crucible match. Stackable doesn't mean that you should use them all together, but that the probability of getting an exotic rises each time you don't get one. Also, try to space them out like 10 or 15 minutes, they seem to have some kind of cooldown, but this is a debated topic I think.

Start a Strike playlist (any one will do). Use 1 3oC at the start. Kill the boss. Look for Exotic drop. Start the next Strike. Use a 3oC. Kill boss. Repeat. Repeat. Repeat. Except for Psion Flayers and Omnigul. They don't work with 3oC. You can also use 1 per Crucible match. Or, do the Iron Tomb mission and use 1 per kill on each of the 3 bosses at the end, then quickly kill yourself and do it repeatedly.