On strong heating of ${\mathrm{H}}_3{\mathrm{PO}}_4$ and ${\mathrm{H}}_3{\mathrm{PO}}_3$ sum of oxidation number of P and B in the final product obtained is:

${\mathrm{H}}_3{\mathrm{PO}}_4\stackrel{∆}{\to }\mathrm{\Delta }{\mathrm{P}}_2{\mathrm{O}}_5(\mathrm{O}.\mathrm{N}:=+5)$

${\mathrm{H}}_3{\mathrm{PO}}_3\stackrel{∆}{\to }{\mathrm{B}}_2{\mathrm{O}}_3(\mathrm{O}.\mathrm{N}.=+3)$

In oxidation state of P in ${\mathrm{H}}_3{\mathrm{PO}}_4$​ is 5 and oxidation state of P in ${\mathrm{H}}_3{\mathrm{PO}}_3$ is 3 ( taking oxidation state of oxygen is -2 and hydrogen +1).

there are  sum is 5+3=8.