Why is ammonium chloride added before addition of ammoniun hydroxide in preciptn of grp 3 cations?

This is to prevent the precipitation of hydroxides of higher groups.

Ammonium chloride is a strong electrolyte. SO, it dissociates completely.

NH₄Cl NH₄⁺ + Cl^-

Now when NH₄OH is added, due to common ion effect its dissociation is supressed.

NH₄OH NH₄⁺ + OH^-

The concentration of hydroxide ion is less. Hence, the ionic product of hydroxide ion and cation only exceeds the solubility product of hydroxides of group 3 cations and they are precipitaed. The solubility product of higher group cations is higher than their ionic product and hence they are not precipitated.