When doing a sneak attack with a bow, what number gets mutiplied by three? Is it the base damage for the bow or is it the damage listed in the inventory which would include all enchantments and potions?

The way I understand it is sneak multipliers apply to total damage. In addition, all your skill, perk, enchantment, and potion multipliers apply to your arrow, even though it doesn't show in your inventory. For instance, if you are using the Bow of the Hunt with a base damage of 10 and Iron arrow with a base damage of 8, and the bow displays 30 damage when you view it in your inventory your total damage is the following:

Bow damage: 30/10 = x3 multiplier

Actual arrow damage: 8 x 3 = 24

Total actual damage: 30 (bow) + 24 (arrow) = 54

The x3 sneak multiplier applies to the 54 total damage for a total of 162. The effect can be witnessed with large opponents that normally take numerous shots to kill. When in sneak they will lose a much large amount of health with one shot, or even be killed in one shot.