For a carbonyl compound (aldehyde or ketone) to exhibit keto-enol tautomerism, it must possess at least one acidic $\alpha$-hydrogen atom attached to an $sp^3$ hybridised carbon adjacent to the carbonyl group ($>C=O$).

The acidity of $\alpha$-hydrogens is due to the strong electron-withdrawing effect of the carbonyl group and the resonance stabilisation of the conjugate base (enolate ion). If compounds I, II, and III all contain oxidizable $\alpha$-hydrogens, they will all exhibit tautomerism.