A triad consists of

A transverse tubule and the two adjacent terminal cisternae constitute a triad.

A sarcoplasmic reticulum has terminal bulged ends called cisterna. It is a storehouse of calcium. T-tubule is an invagination of plasma membrane. In the histology of skeletal muscle, a triad is the structure formed by a T tubule and two terminal cisterna on either side. 

Each skeletal muscle fiber has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. Each skeletal muscle fiber has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally.