The easiest way to balance this chemical equation is to go by ions, not by atoms.
If you write the complete ionic equation for this reaction, you'll get
If you were to eliminate spectator ions, i.e. the ions that are present on both sides of the equation, you'll get the net ionic equation, which looks like this
Notice that you need 3 calcium atoms on the products' side, but only have 1 calcium cation of the reactants' side
Likewise, multiply the phosphate anions by 2 to get them to match the number of phosphate ions present on the products' side.
This will get you
Now take these stoichiometric coefficients and use them in the overall equation
Now you only have to balance the ammonium ions,