You have a semantics problem, not a Color Theory problem. You are confused because you are not using the correct vocabulary to describe different concepts.

A “Triad” is a group of 3 hues.
A “Tertiary” is a mixture of hues.

(A tertiary consists of the mixture of a primary and a secondary hue. Since a secondary is made by mixing two primaries, you have a mixture of 3 hues, called a tertiary.)
Don't confuse these concepts and it's easy as one, two , three!