Three three three

The fourth tennis ball in a can. The bonus mini pen to make a four-pack. The fourth point in a thesis. The fourth project team member. Four downs to score. Ménage à quatre?

Why is #4 such a curse?

I lose less tennis balls when I use a 3-pack can. I hang on to pens longer when there’s just three. People absorb information better in sets of three. My project teams were great with three. Football teams are more likely to get a first down if they don’t get to 4th. Ménage à trois gets weird when you add another.

When isn’t three enough? And optimal? And more efficient?