In 3^rd group cationic analysis, $N{H}_{4}Cl$  is added before adding $N{H}_{4}OH$. This is due to

$N{H}_{4}OH\rightleftharpoons N{H_4}^{+}+O{H}^{-}$                        $N{H}_{4}Cl\to N{H_4}^{+}+C{l}^{-}$

Due to common ion( $N{H_4}^{+}$) effect, the above equilibrium is shifted to backward direction i.e. the     dissociation of $N{H}_{4}OH$ is suppressed. These precipitation of higher groupcations is prevented.