If the two members of a Dobereiner triad are chlorine and iodine, the third member is

The triad is Cl, Br and I.

According to Dobereiner, the atomic mass of the middle element is nearly equal to the arithmetic mean of the other two. 

Therefore, the atomic mass of the third element is $=\frac{35.5+127}{2}=81.25$ which is the atomic mass of bromine.