KH2PO3
Remember is the 'yardstick' of oxygen at '-2'
Since there are 3 oxygens then the oxygen component has an overall oxidation of 3 x -2 = -6
Since K ionises to +1 its oxidation number is '+1'
Since H ionises to +1, and there are two x H , then the oxidation of H2 is +2
And the overall molecule is of 'zero' charge.
Then + 1 + 2 + P - 6 = 0
P - 3 = 0
P = +3 (The oxidation number of phosphorus(P).