To avoid the precipitation of hydroxides of Ni2+ , Co2+ ,Zn2+ and Mn2+ along with those of Fe3+, Al3+and Cr3+ the third group solution should be

The presence of NH4Cl in excess reduces degree of dissociation of NH4OH to a considerable amount, and thus OH⊖ is just sufficient to cross over the ionic concentration product of hydroxides of group III elements to their Ksp.