The "Rule of Three" is used in learning situations with young children, wilderness survival, mathematics, language, and so much more. I want to focus on learning and survival because both of these topics, when looked at from a big picture, also apply to your finances. You need to learn how to use your money efficiently to pay for things that you need to survive (like I said, big picture).
Rule of Three in survival situations:
You can survive for 3 Minutes without air (oxygen) or in icy water
You can survive for 3 Hours without shelter in a harsh environment (unless in icy water)
You can survive for 3 Days without water (if sheltered from a harsh environment)
You can survive for 3 Weeks without food (if you have water and shelter)
The main point of this set of "Rules of Three" that you need to focus on the most substantial problem first. If the biggest problem is that you can't breathe we need to remedy that immediately.
I hear you asking "How does this apply to...