They gave answers in an interview pre rise of iron bro. Rise of iron producer Scot Taylor when asked by eurogamer:
[quote]I'm being told it's my final question. Will Three of Coins be staying for Rise of Iron?
Taylor: Yes, absolutely.
Bungie has never said - what's the best method for using Three of Coins?
Taylor: I always put one on...
Is there a cooldown timer?
Taylor: ...There is not.[/quote]
Source:
http://www.eurogamer.net/articles/2016-09-02-bungie-talks-destiny-rise-of-irons-future-events-wolves-and-raid
English
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What we can get out of this for sure is that there is no cool down. He said he "always puts one on..." which may suggest something about stacking and something about drop rates varying between opportunities. But we can't confirm either of those.
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Well he was asked the best method. The answer he gave to the question was that he only takes 1. Which strongly implies thays his view on the best method. And being the producer, i think he would opt for the correct way. I doubt he'd say he only takes one if he knows that actually stacking more works as a better method. I dunno, i dont really read too much into it. He was asked a few questions and gave clear answers. Case closed for me. But i have nothing against doubters, especially with bungies track record of messing with us lol.