The first thing we have to look at when naming a compound is to determine whether we are naming an ionic compound or a covalent compound. For an ionic compound, we will have a metal and non-metal and for a covalent compound, we'll have two non-metals.
For this compound, In3P, we see that we have a metal and a non-metal so we have to follow the rules for naming ionic compounds. For the cation (always the first element written in the formula), we use the name of the element.
In = indium
The anion (the second element written in the formula) uses the name of the element with the ending changed to -ide
P = phosphorus -> phosphide
Therefore the name of the compound will be indium phosphide.