(I) H₃PO₂ , (II) H₃PO₃ (III) H₃PO₄

Correct increasing order of acidic strength () is

structure of conjugate bases of  I , II , III respectively                                         

          (A)                                        (B)                                         (C)

Consider  (A)                         

                     Equivalent (indistinguishable) structures

                   maximum dispersal of negative charge

consider   (B) :     possibility (I)  

Possibility (II)  

Thus structure (B) is less stable as compared to (A)

Consider (C) :   possibility (I)  

Possibility (II)  

Possibility (III)  

(C) is much more unstable

Thus stability of H₃PO₂^- > H₂PO₃^- > H₂PO₄^-

Other explanation:

-I effect of P=O group is distributed only on one -OH in H₃PO₂ where as on two -OH groups in H₃PO₃

And three -OH groups in H₃PO₄