Step-by-step explanation:
H3PO4 is a weak acid so it partially dissociates in water
Ka1
H3PO4 (aq) + H2O(l) <----> H2PO4-(aq) + H3O+ (aq)
Ka2
H2PO4- (aq) + H2O(l) <----> HPO4 2- (aq) + H3O+ (aq)
Ka3
HPO4 2- (aq) + H2O(l) < ---> PO4 3- (aq) + H3O+ (aq)