The game Dungeons and Dragons uses various kinds of dice. Most are in the shape of the regular polyhedrons (cubes, octahedrons, etc).
When three cube shaped six sided dice are rolled and the total taken, a number from 3 to 18 is found. The distribution of the possible results is a bell shaped curve with an expectation value and peak at 10.5. Thus, the average roll of 3d6 (three six sided dice) is ten and a half (sometimes written as 10-11).
If 1d4, 1d6 and 1d8 were rolled, you'd likewise get a value from 3 to 18. The expectation value is again 10.5.
Is the distribution different? I couldn't figure that part out in my head.