Answer
Its formula is $CrPO_{4}.3H_{2}O$.
Work Step by Step
The cation of chromium(III) phosphate trihydrate has a charge of (III), i.e it is $Cr^{3+}$. The anion is phosphate, i.e $PO_{4}^{3-}$. To balance the charges, 1 $PO_{4}^{3-}$ combines with 1 $Cr^{3+}$. Then it also has 3 (tri) water molecules of hydration. So, its formula is $CrPO_{4}.3H_{2}O$.
