I'm talking about overall chance from needing 20 to needing 1 to hit.
Also +1,67 is bonus of elven accuracy over already present advantage.
Advantage alone gives average +3,32 bonus over normal d20 roll.
elven accuracy gives total +5 bonus over normal 1d20 roll.
____
That's correct. But the question isn't "What is the the average throw of a) 1d20, b) 2d20 drop 1 and c) 3d20 drop 2" but: "My opponent has AC 18. Since I have a +5 proficiency in my weapon, I need a 13 to hit. What is the propability to throw at least one 13 with a) 1d20, b) 2d20 and c) 3d20?"
Answer:
1d20: 40%
2d20: 64%
3d20: 78.4%
Or, to get a full table:
[TABLE="class: grid, width: 500, align: left"]
[TR]
[TD][/TD]
[TD="width: 55"]p 1d20[/TD]
[TD="width: 58"]p 2d20[/TD]
[TD="width: 58"]p 3d20[/TD]
[TD="width: 34"][/TD]
[TD="width: 62"]p+ 2d/1d[/TD]
[TD="width: 57"]p+ 3d/1d[/TD]
[TD="width: 58"]p+ 3d/2d[/TD]
[/TR]
[TR]
[TD="align: right"]1[/TD]
[TD="align: right"]100,00%[/TD]
[TD="align: right"]100,00%[/TD]
[TD="align: right"]100,00%[/TD]
[TD][/TD]
[TD="align: right"]0,00%[/TD]
[TD="align: right"]0,00%[/TD]
[TD="align: right"]0,00%[/TD]
[/TR]
[TR]
[TD="align: right"]2[/TD]
[TD="align: right"]95,00%[/TD]
[TD="align: right"]99,75%[/TD]
[TD="align: right"]99,99%[/TD]
[TD][/TD]
[TD="align: right"]5,00%[/TD]
[TD="align: right"]5,25%[/TD]
[TD="align: right"]0,24%[/TD]
[/TR]
[TR]
[TD="align: right"]3[/TD]
[TD="align: right"]90,00%[/TD]
[TD="align: right"]99,00%[/TD]
[TD="align: right"]99,90%[/TD]
[TD][/TD]
[TD="align: right"]10,00%[/TD]
[TD="align: right"]11,00%[/TD]
[TD="align: right"]0,91%[/TD]
[/TR]
[TR]
[TD="align: right"]4[/TD]
[TD="align: right"]85,00%[/TD]
[TD="align: right"]97,75%[/TD]
[TD="align: right"]99,66%[/TD]
[TD][/TD]
[TD="align: right"]15,00%[/TD]
[TD="align: right"]17,25%[/TD]
[TD="align: right"]1,96%[/TD]
[/TR]
[TR]
[TD="align: right"]5[/TD]
[TD="align: right"]80,00%[/TD]
[TD="align: right"]96,00%[/TD]
[TD="align: right"]99,20%[/TD]
[TD][/TD]
[TD="align: right"]20,00%[/TD]
[TD="align: right"]24,00%[/TD]
[TD="align: right"]3,33%[/TD]
[/TR]
[TR]
[TD="align: right"]6[/TD]
[TD="align: right"]75,00%[/TD]
[TD="align: right"]93,75%[/TD]
[TD="align: right"]98,44%[/TD]
[TD][/TD]
[TD="align: right"]25,00%[/TD]
[TD="align: right"]31,25%[/TD]
[TD="align: right"]5,00%[/TD]
[/TR]
[TR]
[TD="align: right"]7[/TD]
[TD="align: right"]70,00%[/TD]
[TD="align: right"]91,00%[/TD]
[TD="align: right"]97,30%[/TD]
[TD][/TD]
[TD="align: right"]30,00%[/TD]
[TD="align: right"]39,00%[/TD]
[TD="align: right"]6,92%[/TD]
[/TR]
[TR]
[TD="align: right"]8[/TD]
[TD="align: right"]65,00%[/TD]
[TD="align: right"]87,75%[/TD]
[TD="align: right"]95,71%[/TD]
[TD][/TD]
[TD="align: right"]35,00%[/TD]
[TD="align: right"]47,25%[/TD]
[TD="align: right"]9,07%[/TD]
[/TR]
[TR]
[TD="align: right"]9[/TD]
[TD="align: right"]60,00%[/TD]
[TD="align: right"]84,00%[/TD]
[TD="align: right"]93,60%[/TD]
[TD][/TD]
[TD="align: right"]40,00%[/TD]
[TD="align: right"]56,00%[/TD]
[TD="align: right"]11,43%[/TD]
[/TR]
[TR]
[TD="align: right"]10[/TD]
[TD="align: right"]55,00%[/TD]
[TD="align: right"]79,75%[/TD]
[TD="align: right"]90,89%[/TD]
[TD][/TD]
[TD="align: right"]45,00%[/TD]
[TD="align: right"]65,25%[/TD]
[TD="align: right"]13,97%[/TD]
[/TR]
[TR]
[TD="align: right"]11[/TD]
[TD="align: right"]50,00%[/TD]
[TD="align: right"]75,00%[/TD]
[TD="align: right"]87,50%[/TD]
[TD][/TD]
[TD="align: right"]50,00%[/TD]
[TD="align: right"]75,00%[/TD]
[TD="align: right"]16,67%[/TD]
[/TR]
[TR]
[TD="align: right"]12[/TD]
[TD="align: right"]45,00%[/TD]
[TD="align: right"]69,75%[/TD]
[TD="align: right"]83,36%[/TD]
[TD][/TD]
[TD="align: right"]55,00%[/TD]
[TD="align: right"]85,25%[/TD]
[TD="align: right"]19,52%[/TD]
[/TR]
[TR]
[TD="align: right"]13[/TD]
[TD="align: right"]40,00%[/TD]
[TD="align: right"]64,00%[/TD]
[TD="align: right"]78,40%[/TD]
[TD][/TD]
[TD="align: right"]60,00%[/TD]
[TD="align: right"]96,00%[/TD]
[TD="align: right"]22,50%[/TD]
[/TR]
[TR]
[TD="align: right"]14[/TD]
[TD="align: right"]35,00%[/TD]
[TD="align: right"]57,75%[/TD]
[TD="align: right"]72,54%[/TD]
[TD][/TD]
[TD="align: right"]65,00%[/TD]
[TD="align: right"]107,25%[/TD]
[TD="align: right"]25,61%[/TD]
[/TR]
[TR]
[TD="align: right"]15[/TD]
[TD="align: right"]30,00%[/TD]
[TD="align: right"]51,00%[/TD]
[TD="align: right"]65,70%[/TD]
[TD][/TD]
[TD="align: right"]70,00%[/TD]
[TD="align: right"]119,00%[/TD]
[TD="align: right"]28,82%[/TD]
[/TR]
[TR]
[TD="align: right"]16[/TD]
[TD="align: right"]25,00%[/TD]
[TD="align: right"]43,75%[/TD]
[TD="align: right"]57,81%[/TD]
[TD][/TD]
[TD="align: right"]75,00%[/TD]
[TD="align: right"]131,25%[/TD]
[TD="align: right"]32,14%[/TD]
[/TR]
[TR]
[TD="align: right"]17[/TD]
[TD="align: right"]20,00%[/TD]
[TD="align: right"]36,00%[/TD]
[TD="align: right"]48,80%[/TD]
[TD][/TD]
[TD="align: right"]80,00%[/TD]
[TD="align: right"]144,00%[/TD]
[TD="align: right"]35,56%[/TD]
[/TR]
[TR]
[TD="align: right"]18[/TD]
[TD="align: right"]15,00%[/TD]
[TD="align: right"]27,75%[/TD]
[TD="align: right"]38,59%[/TD]
[TD][/TD]
[TD="align: right"]85,00%[/TD]
[TD="align: right"]157,25%
[/TD]
[TD="align: right"]39,05%[/TD]
[/TR]
[TR]
[TD="align: right"]19[/TD]
[TD="align: right"]10,00%[/TD]
[TD="align: right"]19,00%[/TD]
[TD="align: right"]27,10%[/TD]
[TD][/TD]
[TD="align: right"]90,00%[/TD]
[TD="align: right"]171,00%[/TD]
[TD="align: right"]42,63%
[/TD]
[/TR]
[TR]
[TD="align: right"]20
[/TD]
[TD="align: right"]5,00%
[/TD]
[TD="align: right"]9,75%
[/TD]
[TD="align: right"]14,26%
[/TD]
[TD][/TD]
[TD="align: right"]95,00%
[/TD]
[TD="align: right"]185,25%
[/TD]
[TD="align: right"]46,28%
[/TD]
[/TR]
[/TABLE]
As you see, the harder an opponent is to hit, the more important is having advantage or even Elven Accuracy.
If you e.g. have to throw a 13 to hit, Advantage gives you a 60% in the probability to hit and Elven Accuracy gives you another 22.5% on top which mean an overall +96% probability to hit. Thus, your chance to strike your opponent is almost doubled. And the harder the enemies are to hit, the higher this bonus is.