Dobereiner triads are

Dobereiner triads “The atomic weight of middle element is equal to average atomic masses of two elements.

Ex: Li, Na, K Atomic weight of  $\mathrm{Na}=\frac{7+39}{2}=23$

S, Se, Te Atomic weight of  $\mathrm{Se}=\frac{32+125}{2}=79$

Cl, Br, I Atomic weight of $\mathrm{Br}=\frac{355+127}{2}=800$