H3PO4 is not a weak acid. It is a strong acid. Strong acids completely dissociate in water, releasing all of their hydrogen ions (H+) and leaving no undissociated acid molecules. H3PO4 is a triprotic acid, meaning it can donate three protons (H+ ions). In water, it dissociates completely into three hydrogen ions and one phosphate ion (PO4^3-):

H3PO4 (aq) + 3H2O (l) → 3H3O+ (aq) + PO4^3- (aq)