In aminobenzoic acid, do we have three different pKa's?
The two acidity constants in 4-aminobenzoic acid are due to the loss of $\ce{H^+}$ from the protonated form of 4-aminobenzoic acid (the 4-carboxyphenylammonium cation), transforming it into 4-aminobenzoic acid, and then a further loss of $\ce{H^+}$ turning it into the 4-aminobenzoate anion.



The first two ionizations have a $\mathrm{p}K_\mathrm{a}$ of around 5, while the next ionization quite likely has a $\mathrm{p}K_\mathrm{a}$ well below ~25 (compare with other substituted anilines), so you won't find any appreciable amount of deprotonated nitrogen atoms in an aqueous solution. Groups with N−H bonds are far less acidic than their O−H counterparts due to the lower electronegativity of the nitrogen atom, which is less capable of stabilizing the extra negative charge.