Substitute 3d6 System

I like the idea of learning to play paper and pencil rpgs by substituting a d20 with three standard six sided dice (3d6) because they're very common and you might not always have access to your fancy specialty dice. In other words, a 3d6 system. There are some differences between a d20 and 3d6 that will surprise and confuse you if you don't review the math. Refer below to this magnificent chart I made based on probability of outcomes rolling 3d6. Rate means rate of meeting or beating the result.

Let me simplify it like this:

1. An easy challenge is an 8.
2. A medium challenge is a 10.
3. A hard challenge is a 12.

But choosing a target number is not that simple. 3d6 is unlike the d20 which essentially makes 5 easy, 10 medium, and 15+ hard. Be aware, with 3d6, moderate results (8 to 13) become common and extreme results (3 to 6 and 15 to 18) become rare. Rolls are more likely to barely succeed or barely fail. Dice rolls are more predictable and chance is less of a factor.

Target Numbers
Pick a target number for a skill check based on the probability of success and adjust based on the player's modifier rather than an arbitrary DC based on easy, medium, hard. For instance, let's say your player is performing a skill check. You determine that the task should be challenging such that they have a 50/50 chance to succeed. A 10 is the same as 50/50. They have a +3. Tell them to roll 13 to succeed. Bear in mind, someone without any bonuses will have an approximate 17% success rate performing a skill check with a target number of 13.

A Game with Small Modifiers

To play an rpg by substituting 3d6 for a d20, you need to re-scale all the numbers including armor class, challenge rating, bonuses, penalties, and other modifiers because any small modifier matters a lot.  You'll probably have to adjust numbers impromptu too. A +2 is enough to make a hard challenge into a medium challenge and a +4 is enough to make a hard challenge easy.

Critical Success and Critical Failure

You have an approximate 4.5% chance of rolling a 5 or lower and likewise to roll a 16 or higher; therefore, treat a natural 3, 4, or 5 as critical failure and treat a natural 16, 17, or 18 as a critical success.

Advantage and Disadvantage

Rolling a 4d6 drop the lowest / highest is going to feel like the 5e advantage system, but that is going to increase rolls by an average of 3.5 which is substantial. Consider assigning a more modest +2 bonus for advantage and a -2 for disadvantage.

 

d66 for Tables: 

Roll 2d6 for a 6 base percentile die. Treat one d6 as a tens place and one d6 as the ones place, and you can roll any number from 11 to 66 excluding numbers with 7, 8, 9, and 0. You can roll 36 different results.