Rule of Threes, The

The Rule of Threes is a cosmic principle presented in the original Planescape Campaign Setting.

It is based on something that seems to be a phenonomen in the multiverse, that everything comes in threes: there are three cosmic truths (Center of the Multiverse, Unity of Rings, and Rule of Threes), three transitive planes (Astral, Ethereal, and Shadow) and three levels of existence (Outer Planes, Inner Planes, and Prime Material). Between any two extremes (which the Opposition focus on) is a median that reconciles the other two: between Law and Chaos is balance, between Good and Evil is neutrality, between Fire and Earth is Magma, between the celestials and fiends are the three neutral races: modrons, slaadi, and rilmani.

See also the Unity of Rings and Center of the Multiverse.

Source: Planescape Campaign Setting.