The d3 is a three sided die used often in roleplaying games like Dungeons and Dragons. It is shaped like a gem; three sides extend all the way from one end to the other creating a shape somewhat similar to a sunflower seed.
Unlike its brothers (d4, d6, d8, d10, d12, d20 and others) the d3 isn't used so much for hit dice, confirming whether attacks hit or miss or how hard they hit if they do, but its meaning is rather to indicate either amounts of time or amounts of special damage. Also, the damage potential of smaller creatures or weapons can be governed by these d3 dice.
Incubation times of specific diseases are often declared by the rolling of one or more d3 dice. Additionally, a modifier can be added or subtracted from the total. For instance, 1d3+1 days means a minimum of 2 and a maximum of four days. The d3 is commonly used to generate the amount of days, weeks or months prior to important events.
d3's are often rolled when a RPG character is the victim of attacks drawing special damage, be it delayed or not. Most predominant is Ability Damage, which is damage to one or more of a character's main attributes: Strength, Dexterity, Constitution, Intelligence, Wisdom, Charisma. 1d3 Con Damage for example means minimally 1 and maximally 3 points of damage to one's Constitution