Answer
Please see the work below.
Work Step by Step
We know that
$\frac{ 3NH_4^{+}ions\space atom}{1(NH_4)_3\space PO_4unit}\times \frac{4\space H \space atoms\space unit}{1NH_4^{+}- ions}\times \frac{1(NH_4)_3PO_4\space unit}{4\space O \space atoms}=\frac{12H\space atoms}{4\space O atoms}=\frac{3}{1}$
