The order of relative stability of the contributing structure is:...

The factors that affect the stability of a species are:
Number of Covalent Bonds: More the number of covalent bonds, more will be the atoms with octet, more will be the stability.
Formal Charge: The species should have the least formal charge to be stable.
Charge Separation: The species should have least charge separation to be stable.
Charge on Atoms: The species with negative charge on electropositive atom and positive charge on electronegative atom will be least stable.

$I > II$, since neutral resonating structures are more stable than charged resonating structures.$II > III$, since the stability of the structure with –ve charge on more electronegative atom is higher.

Also, in structure I, the octet of every element is complete.