On heating, phosphorous acid disproportionates: $4\mathrm{H}{}_3\mathrm{PO}{}_3\to 3\mathrm{H}{}_3\mathrm{PO}{}_4+\mathrm{PH}{}_3$. The ratio of the number of P atoms oxidised to the number reduced is

P in $\mathrm{H}{}_3\mathrm{PO}{}_3$ is +3. In $\mathrm{H}{}_3\mathrm{PO}{}_4$ it is +5 (oxidised, loses 2 electrons each) and in $\mathrm{PH}{}_3$ it is -3 (reduced, gains 6 electrons). Electron balance: 3 x 2 = 1 x 6, so 3 P are oxidised for every 1 reduced.