Arrange hypophosphorous acid $\left({\mathrm{H}}_3{\mathrm{PO}}_2\right)$, phosphorous acid $\left({\mathrm{H}}_3{\mathrm{PO}}_3\right)$ and Phosphoric acid $\left({\mathrm{H}}_3{\mathrm{PO}}_4\right)$ in the decreasing order of acidic strength

There is very little difference in acid strength in the series ${\mathrm{H}}_3{\mathrm{PO}}_4,{\mathrm{H}}_3{\mathrm{PO}}_3\text{ and }{\mathrm{H}}_3{\mathrm{PO}}_2$ because the hydrogen in these acids are not all bonded to phosphorus. In the above three acids although the number of -OH groups (ionizable hydrogen increases, yet the acidity does not increase very much.