TEXT: REPLACING THE D20 WITH MULTIPLE DICE IN AD&D 
   
  Metagame Analysis: The Bell Curve 
  In general, rolling either 3d6 or 2d10 leads to a grittier game, because 
  there will be far fewer very good or very bad rolls.  
   
  Example:  When rolling 3d6 you can no longer roll 1, 2, 19 or 20, and 
  most rolls will be clustered around the average of 10.5. With a d20, 
  every result is equally likely; you have a 5% chance of rolling an 18 
  and a 5% chance of rolling a 10.  
   
  With 3d6, there’s only one possible combination of dice that results 
  in an 18 (three sixes, obviously), but there are twenty-four 
  combinations that result in a 10.  
   
  Players used to the thrill of rolling high and the agony of a natural 1 will 
  get that feeling less often — but it may be more meaningful when it does 
  happen. Good die rolls are a fundamental reward of the game, and it 
  changes the character of the game when the rewards are somewhat 
  stronger but less frequent. 
   
  Game balance shifts subtly when you use multiple dice instead of a 
  single d20. Rolling multiple dice gives you more average rolls, which 
  favors the stronger side in combat. In the AD&D game, that’s almost 
  always the PCs. Many monsters, especially low-Hit Dice monsters 
  encountered in groups, rely heavily on a lucky shot to damage PCs. 
  When rolling multiple dice, those lucky shots are fewer and farther 
  between. In a fair fight when everyone rolls a 10, the PCs should win 
  almost every time. Using multiple dice means that results adhere more 
  tightly to that average. 
   
  Another subtle change to the game is that rolling either 3d6 or 2d10 
  instead of 1d20 awards bonuses relatively more and the die roll relatively 
  less, simply because the die roll is almost always within a few points of 10. 
  A character’s ability score modifiers, proficiency and common ability 
  bonuses, attack bonuses, and magical adjustments have a much 
  bigger impact on success and failure than they do in the standard d20 
  rules. 
   
  The following charts illustrate the probabilities of each result when rolling 
  a single d20, 3d6, or 2d10: 
   
  1d20 Results (Average Roll = 10.5) 
  Result 
  Percentage 
  Result 
  Percentage 
  1 
  5.00 
  11 
  5.00 
  2 
  5.00 
  12 
  5.00 
   
  3 
  5.00 
  13 
  5.00 
  4 
  5.00 
  14 
  5.00 
  5 
  5.00 
  15 
  5.00 
  6 
  5.00 
  16 
  5.00 
  7 
  5.00 
  17 
  5.00 
  8 
  5.00 
  18 
  5.00 
  9 
  5.00 
  19 
  5.00 
  10 
  5.00 
  20 
  5.00 
   
    
  3d6 Results (Average Roll = 10.5) 
  Result 
  Percentage 
  3 
  0.46 
   
  4 
  1.39 
   
  5 
  2.78 
   
  6 
  4.63 
   
  7 
  6.94 
   
  8 
  9.72  
  9 
  11.57 
    
  10 
  12.50  
  11 
  12.50 
    
  12 
  11.57 
    
  13 
  9.72 
    
  14 
  6.94  
  15 
  4.63 
    
  16 
  2.78  
  17 
  1.39  
  18 
  0.46 
   
    
  2d10 Results (Average Roll = 11.0) 
  Result 
  Percentage 
  2 
  1.00 
    
  3 
  2.00 
    
  4 
  3.00 
    
  5 
  4.00 
    
  6 
  5.00 
    
  7 
  6.00 
    
  8 
  7.00 
    
  9 
  8.00 
    
  10 
  9.00 
    
  11 
  10.00 
    
  12 
  9.00 
    
  13 
  8.00 
    
  14 
  7.00 
    
  15 
  6.00 
    
  16 
  5.00 
    
  17 
  4.00 
    
  18 
  3.00 
    
  19 
  2.00 
    
  20 
  1.00 
   
   
  Automatic Successes and Failures 
  Automatic successes and automatic failures occur as follows: 
   
  3d6:  Automatic successes occur on a natural 18 and automatic failures 
  on a natural 3. Neither occurs as often as in standard d20 die roll (less 
  than ½% of the time as opposed to 5% of the time). 
   
  2d10:  Automatic successes occur on a natural 20 and automatic failures 
  on a natural 2. Neither occurs as often as in standard d20 die roll (less 
  than 1% of the time as opposed to 5% of the time). 
   
  Critical Hits and Fumbles 
  The rules for automatic successes and failures given above could apply 
  for attack rolls as well.  This, however, would make critical hits and 
  fumbles rare in combat; and would undermine the effectiveness of 
  magical weapons such as a dagger of venom or vorpal sword. 
   
  Rather than use the rules given for automatic successes and failures 
  above, the following rules may be used so that both critical hits and 
  fumbles factor into combat with greater frequency: 
   
  3d6:  Critical hits occur on a natural 16-18 and fumbles on a natural 3-5. 
  Both would occur as often as in standard d20 die roll (4.63% of the time 
  as opposed to 5% of the time). 
   
  2d10:  Critical hits occur on a natural 19-20 and fumbles on a natural 2-
  3. Neither occurs as often as in standard d20 die roll (3% of the time as 
  opposed to 5% of the time). 
   
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/AD&D 3e/AD&D 3e Sheets & Supplements/AD&D 3e Optional Rule Replacing the d20 with Multiple Dice in AD&D.pdf