I need to find the Positive and negative ions for lithium phosphide. Also, is the formula for that Li3P?

Answer 1

Lithium phosphide is a salt with a lithium cation and a phosphide anion.

Li is in Group 1. How many electrons does it have to lose to get to the configuration of the next lowest noble gas? That tells you the (positive) charge on the lithium cation.

What group is phosphorus in? How many electrons does it need to get up to the number of the next noble gas? That tells you the (negative) charge on the phosphide anion.

If you have the right number of lithiums for each phosphide, the charges will cancel and you will have the correct formula.

And yes; the formula you suggest is correct.