Three of coins have been shown to favor a time of 7-8 minutes of being active to drop an exotic. This is an average time for a strike. So just run strikes and use 1 three of coins each time, or stack them at once to improve our chances at one engram. However, if you have over 30 or so, you should just farm off a boss with them disregarding the 7-8 minute "cooldown" bc they have a chance to drop then. A few farming spots are, The gate lord on Venus in the mission, "Eye of A Gate Lord", The three sol progenies at the end of the mission, "The Black Garden", and the three Iron Lords in the mission, "The Iron Tomb". Don't forget to orbit or die as soon as you kill the boss to save your checkpoint. TL;DR- Three of coins with strike and mission bosses. Save your checkpoints too. Hope this helped.