The law of triads is applicable to a group of

Cl, Br, I follow law of triads.

Atomic weight of $Cl=35.5$ .

Atomic weight of $\text{Br}=80.0$

Atomic weight of $\text{I}=127$

According to law of triads the arithmetic mean of first and third elements is equal to atomic weight of middle element.

Arithmetic mean of $\text{C}l\text{and}\text{I}=\frac{35.5+127}{2}$ $=81.25$

So, Cl, Br, I nearly follows law of triads.