Rule of three or Rule of Thirds may refer to:

• Rule of three (aeronautics), a rule of descent in aviation
• Rule of three (C++ programming), a rule of thumb about class method definitions
• Rule of three (computer programming), a rule of thumb about code refactoring
• Rule of three (hematology), a rule of thumb to check if blood count results are correct
• Rule of three (mathematics), a method in arithmetic
• Rule of three (medicinal chemistry), a rule of thumb for lead-like compounds
• Rule of three (statistics), for calculating a confidence limit when no events have been observed
• Rule of threes (survival), the rule of threes involves the priorities in order to survive

• Rule of Three, a podcast by Jason Hazeley and Joel Morris
• Rule of Three, a series of one-act plays by Agatha Christie
• The Bellman's Rule of Three in The Hunting of the Snark, a poem by Lewis Carroll
• The Rule of Thirds, a 2008 album by Death In June

• Rule of threes (survival), the priorities in order to survive
• Rule of Three (Wicca), a religious tenet
• Rule of three (writing), a principle of writing, and of rhetoric

• Rule of thirds, a rule of thumb for composing visual images
• Rule of thirds (diving), a rule of thumb for divers
• Rule of thirds (military), a rule of thumb in military planning

• All pages with titles beginning with Rule of th
• Three-sigma rule, for a normal distribution in statistics
• Triumvirate, a political regime dominated by three powerful individuals