A triad is three things or people considered as one unit. A triad is a trio.
A triangle is a shape with three sides. Similarly, a triad is a set of three things or people. To be a triad, three individual parts must also be considered a unit. A band with three members is a triad. A business with three divisions is a triad. A family with three members is a triad. A book with three volumes is a triad (usually called a trilogy). Triads are also called threesomes and trios.