I used over 300 three of coins since it came out and I've found that I obtained a 20% drop rate (using one at a time). I do not have much data on stacks but I do know I used two at a time, 10 times and received 3 (30%). Hope this helps. Note: 75% of them were duplicates (I thought Bungie said they fixed this!?!) so I have about 20 new exotic shards from it.