I came, I saw, I conquered. Father, Son, Holy Spirit. Lost sheep, lost coin, lost son. Niña, Pinta, Santa Maria. 3D. Rock, paper, scissors. 3 point basket. 3 ring circus. Hear no evil, see no evil, speak no evil. On your mark, get set, go. The Good, the Bad, and the Ugly.
Likewise with weight lifting. If you want to get strong, the "3" approach works well:
3 reps per set.
3 sets per exercise.
3 exercises per body part.
3 minutes of rest between sets.
3 workouts per week.
Remember: The triangle is the strongest geometric shape.