According to Dobereiner, the atomic weight(or the atomic mass) of the central element is nearly or exactly equal to the mean of the atomic weights of the other two elements in that triad.
In the case of Ca ( at. wt. = 40.0)
Sr ( at. wt. = 87.6)
Ba ( at. wt. = 137.3)
The atomic weight of the central element, Sr = (40 + 137.3)/2
As this calculated value(=88.6) of Strontium is near to its actual atomic weight value(=87.6) ,Dobereiner suggested that Calcium , Strontium and Barium were also a triad.