In an aqueous solution of phosphoric acid ($K_{a1}=7.5\times 10^{-3}$, $K_{a2}=6.2\times 10^{-8}$, $K_{a3}=4.8\times 10^{-13}$), the species present in the smallest concentration is

Each successive ionisation is weaker by several orders of magnitude and is suppressed by the $\mathrm{H}{}^{+}$ already present. $K_{a3}$ is the smallest, so $\mathrm{PO}{}_4{}^{3-}$ has the lowest concentration; $[\mathrm{HPO}{}_4{}^{2-}]\approx K_{a2}$ in magnitude.