In[63]:= Partition[Extract[ivv, Flatten[{{#, 1, 1}, {#, 2, 3}} & /@ Range[3], 1]], 2] Out[63]= {{a[1, 1, 1], b[1, 2, 3]}, {a[2, 1, 1], b[2, 2, 3]}, {a[3, 1, 1], b[3, 2, 3]}}
