There's a theory being mooted that ties in with the name 3oC. Do 2 strikes. You'll have the streak buff active. Pop a 3oC at the start of 3rd strike. Then do another 2 strikes & pop another 3oC. So basically pop one every 3 strikes, tying in with the 3 in the name. Some people have reported a good success rate of exotic drops using this method.

I am not sure if it's me but I seem to mostly get exotics when I pop the 3oC right before the boss dies like around 1-3% hp

"Stackable" is LINEAR. It's chronological. You play one 3oC. Kill an ultra. If that doesn't pay an engram then you do it all over again. This next time the 1st 3oC has a "lingering" effect that increases your chances next time. Carry on doing this and after 7 or 8 3oC (on average) you will get an engram. The "nerf" that was introduced was simply a delay timer so that you can't play them back to back after only a few seconds/minutes (as those repeatedly killing Draksis or Zydron were doing). The stacking and the return rate are the same - but there has to be a reasonable time between each 3oC that you play (some have speculated it might be 10 minutes). It does NOT help to use more than one 3oC at once. That is NOT what "Stacking" means.

Don't use say 5 at one time. That is not how they stack. they already explained this. How they stack is you use one, kill an ultra and if nothing drops use another one to kill another ultra. and this is where it stacks. say first time you had a 1 out of 10 chance. get nothing then use another 3oc and now you have a 2 out of 10 chance of it dropping. and this keeps stacking until you get one. its a very high chance once you get to 6 or 7 ultras. But they were being abused so what they did the made it so you have to have a sufficient amount of time between ultra kills. Strike bosses work because it takes time to get to them. But say popping one and killing the first bond brother and he drops nothing then popping another one to kill the second is not a good idea. not enough time in between them. I hope this makes sense. of not I hope I can try and clear it up

If you use one against against an ultra and didn't get an exotic. The next time you use a 3oC, your chances of an exotic is higher. Usually people use 3oC during strikes. I'm not sure if the following statement is true so don't quote me. Bungie mentioned that the longer you run a strike without going to orbit (meaning none of your fireteam leaves when you go to the next strike) your reward will get better. I'm not sure if the 3oC work like this too. But I've been doing this with fireteam and I've been getting exotic drops one in 4-5 3oC use.

Don't stack man, not worth it.