n-Factor of phosphorous acid
$n$-Factor is basically a method to find out a relationship between the compound and what it is equivalent to in terms of acidic nature or basic nature. Note that the two hydrogen atoms in $\ce{H2SO4}$ are both attached to oxygen. But in $\ce{H3PO3}$ two of the three hydrogens are attached to oxygen and the other hydrogen is directly attached to phosphorous.

Note that only those hydrogens attached to highly electronegative elements can be donated easily in case of inorganic acids. One could explain better by using the concept of stabler structures with the $\ce{H}$ removed from $\ce{O}$ than from $\ce{P}$ by resonance. But for now know it as a thumb rule.

Therefore, since $\ce{H3PO3}$ has only two $\ce{H}$ attached to $\ce{O},$ its $n$-factor is 2 and not 3.