A number is divisible by 3 iff the sum of its digits is divisible by 3. Of course, you can use this rule again to decide about the sum of the digits, if it's still too large. In fact, it is true that the remainder of the digit sum after division by 3 is equal to the remainder of the number.

Actually, a similar rule can be formulated for any divisor (although it won't be nearly as elegant); see the set of decimal representations of numbers divisible by 17 is regular for an example.

C++ has its own "rule of three": These three methods

go together. If you write one, you probably need to write all three!

Why? You need one of these methods when your object "owns" something. If an object owns something, care must be taken in copying it and assigning it (do we want to end up with two objects owning the same thing?) and in destroying it (what happens to what the late object used to own?). Interestingly, almost all uses for these 3 methods is related to ownership. So if you need to write one, you (probably) need to write all three.

Rule of Three

The Marine Corps uses what is called the 'Rule of Three' as a general principle of management. Over many years of trial and error, the people in charge have found that most men can only keep track of three things at once. To those of you who may be thinking "Bah, I'm better than Marines then. Look at me: chewing gum, talking to friends, reading this node, and hurridly typing my nasty reply! That's four!" Well, this is three things during a combat situation; when you're shooting and being shot at. I consider it rather generous.

According to the Rule of Three, all leaders should only be directly in charge of three subordinates. At the bottom level is the Rifleman, who isn't in charge of anyone. After that, there is the Fireteam Leader, who is in charge of three Riflemen. The Squad Leader is in charge of three Fireteam leaders; the Platoon Sergeant is in charge of three Squad Leaders, and there are three platoons in a company.

Those with military experience will see some small flaws in the practice of the Rule of Three (namely, the fourth, headquarters platoon which is in my company), but in general it is adhered to, and works very well.
While not commonly heard of, the Rule of Three is a standard of measurement to determine how long a person can live under certain circumstances. This rule is used mainly in forensics and for military purposes. Basically, the purpose of the rule is to set a general guideline as to how long a person can survive without Air, Water, & Food and derived its name from the abundance of threes in the results.

Averages:
- Air = 3 Minutes
- Water = 3 Days
- Food = 3 Weeks

The rule regarding water is based on a person that is fully hydrated at the beginning of deprivation. If you are already dehydrated the time to live will be increasingly shorter. It is also important to note that in cooler temperatures the time to live will be closer to 4 days due to a reduction in body perspiration.

I hope never to find myself in a situation where I need to know this, and hope the same goes for you. I do know that in certain military situations this information is used to allow a soldier to prioritize his/her objectives when stranded or in a situation without food and water.