d2 = {{3, 2}, {4, 1}, {5, 6}}
a = {18, 32, 50}
d3 = Table[{d2[[n, 1]], a[[n]], d2[[n, 2]]}, {n, 1, 3}]
This will yield
3 18 2
4 32 1
5 50 6
In[1]:= d = {{3, 2}, {4, 1}, {5, 6}}; In[2]:= d /. {a_, b_} -> {a, 2*a^2, b} Out[2]= {{3, 18, 2}, {4, 32, 1}, {5, 50, 6}}
