Ion chromatography of anions
In ion chromatography there are multiple retention mechanisms, the ion-exchange is the primary one. However, let us keep it simple.

You should be able to explain why monovalents retain less than divalents? Use Coulomb's law! The force of attraction between the ion and the ion-exchange on stationary phase is proportional to the product of their charges and inversely proportional to the hydrated radius. Think about it why fluoride comes out before chloride, almost universally on all IC columns.

Another complication, this explanation breaks down with large anions like iodide, thiocyanate and perchlorate. They retain extremely long on most IC columns. They are called polarisable ions. These ions are sticking because of some other reasons.

Coming to divalents and trivalents: The retention of divalents and trivalents are very complicated. It depends on the chemistry of the stationary phase and its cross-linking and the very nature of the charged site on the ion-exchanger. However you can do a ball-park at an elementary level. Hard to predict in general.

Is phosphate ion always 3(-) or it can have 2(-) or 1(-) charge? Is the ionization pH dependent? The pKa (third proton) is >12. Is the mobile phase pH above 12?