giant.robot
Adventurer
To put some of the numbers in perspective:
Say you're trying to roll against a DC15 on a D20. With no modifiers you need to roll at least a 15 on your d20 to succeed, you only have a 30% change of rolling a 15 or higher on a d20. Now say you've got a +5 modifiers on the roll, you need to roll at least a 10 on the d20 to succeed which you have a 55% chance of doing. Every positive modifier you have to your d20 roll is an additional 5% likelihood that you will succeed in the roll. You always have a 5% chance of rolling a critical failure or success.
Moving to 3d6 is a totally different kettle of fish. With no modifiers you only have a 9.26% chance of rolling a 15 with 3d6. That means rolling a DC15 on 3d6 has about the same statistical likelihood as rolling a DC19 on a d20. However rolling a DC10 on 3d6 is actually easier than on a d20 because there's more combinations of dice that add up to at least 10 when you have 3d6. Values from 3d6 rolls form a bell curve, some values of rolls are more likely than others because a larger permutation of dice adds up to that value. Modifiers to 3d6 rolls no longer increase your odds of success linearly due to the curved nature of the dice permutations.
Switching from d20 to 3d6 means you're going to have a much more difficult game since rolls considered moderate difficulty in d20 parlance are extremely difficult in 3d6. Also as mentioned, your modifiers need to be reduced because something as seemingly benign as a +2 bonus really throws off the difficulty curve.
Simply wanting to be different is a bad reason to completely break your game. Everything in the d20 system was written based on the probability curves of a d20, by moving to 3d6 you throw that all completely out the window. Unless you go through and rebalance absolutely everything in the game (attack bonuses, skill point totals, ability bonuses, etc) you're going to end up with very unhappy players. A 1st level Fighter against a 1st level Goblin (AC15) needs to roll at least a 10 (+1 BAB, +4 Str mod) to hit it. In d20 they have a 55% chance while with 3d6 they have a 62.5% chance. However if that Goblin was AC17 instead they would need to roll at least a 12. That's a 45% chance with a d20 but only a 37.5% chance with 3d6.
Say you're trying to roll against a DC15 on a D20. With no modifiers you need to roll at least a 15 on your d20 to succeed, you only have a 30% change of rolling a 15 or higher on a d20. Now say you've got a +5 modifiers on the roll, you need to roll at least a 10 on the d20 to succeed which you have a 55% chance of doing. Every positive modifier you have to your d20 roll is an additional 5% likelihood that you will succeed in the roll. You always have a 5% chance of rolling a critical failure or success.
Moving to 3d6 is a totally different kettle of fish. With no modifiers you only have a 9.26% chance of rolling a 15 with 3d6. That means rolling a DC15 on 3d6 has about the same statistical likelihood as rolling a DC19 on a d20. However rolling a DC10 on 3d6 is actually easier than on a d20 because there's more combinations of dice that add up to at least 10 when you have 3d6. Values from 3d6 rolls form a bell curve, some values of rolls are more likely than others because a larger permutation of dice adds up to that value. Modifiers to 3d6 rolls no longer increase your odds of success linearly due to the curved nature of the dice permutations.
Switching from d20 to 3d6 means you're going to have a much more difficult game since rolls considered moderate difficulty in d20 parlance are extremely difficult in 3d6. Also as mentioned, your modifiers need to be reduced because something as seemingly benign as a +2 bonus really throws off the difficulty curve.
Simply wanting to be different is a bad reason to completely break your game. Everything in the d20 system was written based on the probability curves of a d20, by moving to 3d6 you throw that all completely out the window. Unless you go through and rebalance absolutely everything in the game (attack bonuses, skill point totals, ability bonuses, etc) you're going to end up with very unhappy players. A 1st level Fighter against a 1st level Goblin (AC15) needs to roll at least a 10 (+1 BAB, +4 Str mod) to hit it. In d20 they have a 55% chance while with 3d6 they have a 62.5% chance. However if that Goblin was AC17 instead they would need to roll at least a 12. That's a 45% chance with a d20 but only a 37.5% chance with 3d6.
