What is the name of #Cu_3P#?

We're dealing with an ionic compound, so what you want to do is name the cation (metal) first. The name of the metal is always taken straight from the periodic table. Cu is the atomic symbol for the element copper. Thus that is what comes first.

The charge of the metal (which is 1 in our case) is written in Roman numerals and is placed in parentheses to indicate that the element is a transition metal. Transition metals can assume more than one charge, so it's just easier to be more specific when stating the charge of a transition metal.

For example, copper can have a 1+ charge or a 2+ charge.

Now the anion (nonmetal) is phosphorus, but you have to take the root word and change the ending to -ide. Therefore, phosphorus becomes phosphide.

In summary, when naming ionic compounds, the procedure goes like this:

name of first element (roman numeral of charge in parentheses if dealing with a transition metal) + root name of second element that has the suffix -ide

Cu3P stands for copper phosphide.

The name of Cu3P is copper(I) phosphide.