Another comparison graph: if you leave advantage and disadvantage alone, here's the effect of advantage on the chance of success for different thresholds, compared to the effect relative to the ordinary 1d20. (The success chances with advantage are the same, but we're boosting them from a different baseline). Note that with the 3d20 keep middle base roll, advantage becomes somewhat less valuable on easy rolls, and somewhat more valuable on difficult rolls, being the equivalent of a +6 on rolls when you need a natural 14 or 15 to succeed. I actually kind of like that, since it encourages characters who don't have large bonuses to find sources of advantage to overcome difficult challenges, still allowing them a good chance of success if they can get it. For example, a character with a low athletics bonus trying to make a difficult climb gets even more mileage out of a climber's kit with this system.
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