The 3 rule in Sudoku refers to the three main constraints that must be satisfied in every valid Sudoku puzzle.
The Three Rules
1. Each row must contain all numbers 1-9 exactly once
2. Each column must contain all numbers 1-9 exactly once
3. Each 3x3 box must contain all numbers 1-9 exactly once
Why These Rules Matter
These three rules form the foundation of all Sudoku solving techniques and strategies.