Gary Gygax has only three abilities − Physical, Mental, and Spiritual − and each is defined by its two subdivisions.Physical, Mental, Spiritual each break down into two subtraits and each of those breaks down further into a Capacity, a Power, and a Speed -> 18, not 3.
How reach of them related to skills and attribute checks means they aren't really just 3.
The tertiary subdivisions (‘attributes’) are just numbers that ‘measure’ the proficiency and speed of the ability. They dont change the meaning of the ability.
For example, when Gary Gygax lists the skills for each ability, the skill lists are three: Physical, Mental, and Spiritual.
Not 6. Not 18. But 3.
His gaming system is an ability threesome.