The formula for nickel(III) phosphide is Ni₃P. Here's how to figure that out:

* Nickel(III) means the nickel ion has a +3 charge (Ni³⁺).

* Phosphide refers to the phosphide ion, which has a -3 charge (P³⁻).

To balance the charges, you need three nickel ions (3 x +3 = +9) and one phosphide ion (-3). This gives you the formula Ni₃P.