- H is +1.

- O is -2.

- P is x.

The overall charge of the ion is -1.

So, we can set up the equation:

(+1)(2) + (-2)(3) + x = -1

2 - 6 + x = -1

x = -1 + 4

x = +3

Therefore, the oxidation number of P in H2PO3- is +3.