The equilibrium constants for the reactions are:
H3PO4K1⇌H++H2PO−4;K1H2PO−4K2⇌H++HPO2−4;K2HPO2−4K3⇌H++PO3−4;K3
The equilibrium constant for, H3PO4⇌3H++PO3−4 will be:
K1×K2×K3
K1=[H+][H2PO−4][H3PO4];K2=[H+][HPO2−4][H2PO−4]
K3=[H+][PO3−4]HPO2−4
Multiplying these three
K1×K2×K3=[H+]3[PO3−4][H3PO4]