There is no chance of this. Three of coins just activate the exotic particles buff which, when triggered, rolls the dice on whether or not it will cause an exotic to drop. 1) using multiple at a time does nothing past the first one. Activating one while the buff is active does nothing. Never use a second one until the buff is gone from the character screen. When it says they stack, it means that if you don't get an exotic from one, the next one will have a higher chance. 2) the exotic particles buff only triggers when killing an ultra or finishing a crucible match. The blade won't even trigger them. 3) the buff in no way affects any other drops. It does absolutely nothing but provide that shot at an exotic drop when triggered. The three of coins have been extensively tested by the community. Edit: and for the record, it doesn't do anything for the psion cloaks either. The flayers don't trigger the coins either.