I used thousands of 3oC in D1, and became an expert exotic engram farmer. I even taught other gamers how to farm exotic engrams quickly. Prior to the release of AoT, I had saved 150 exotic engrams in the vault in case Bungie chose to increase the light level with their final update for the game. Based on my experience with thousands of 3oC, the drop rate is one exotic engram for every ten 3oC in D1. Ten exotic engrams can be obtained in 60 - 90 minutes from about 100 3oC, depending on long you want to play. All you have to do is farm the Iron Tomb mission. In D2 the best way of obtaining exotic engrams was public events until they got nerfed. There is no clear method of obtaining these engrams quickly anymore, but many gamers have all the exotic engrams they really want anyway. D1 and D2 are adaptive, as you mentioned. The game monitors which weapons you use most often and drops these weapons the least often. This slows the progress of leveling up gear, especially weapons.