To tell if (NH4)3P, Ammonium phosphide, is ionic or covalent (also called molecular) we we need to recognize NH4 + is a polyatomic ion and has a charge of 1+. The 1+ charge applies to the entire Ammonium ion.

The Nitride ion is P 3- overall ionic charge of 3-. When we have a the polyatomic ion NH4+ and a negative ion, like P 3-, the compound is considered ionic.

Because the Ammonium ion, NH4 + and P 3- have opposite ionic charges, they attract and form an ionic bond. So in answer to our question (NH4)3P is an ionic compound. Do note that the bonds in the NH4 + polyatomic ion are covalent since they consist of only non-metals. But the overall compound, (NH4)3P is an ionic compound.
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Helpful Resources

Metals, Non-Metals on the P- Table: https://youtu.be/OoooStZQHdA
Ionic, Covalent, & Polar Covalent: https://youtu.be/OHFGXfWB_r4
Electronegativity for each element: https://en.wikipedia.org/wiki/Electronegativity
Memorizing Polyatomic Ions: https://youtu.be/vepxhM_bZqk
Finding Ionic Charge: https://youtu.be/N4N1Njh7nCo
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In general, ionic compounds:

- form crystals.
- have high melting points and boiling points.
- are hard and brittle.
- conduct electricity when dissolved in water.
- as solids do not conduct electricity.

For more chemistry help, see http://www.Breslyn.org.