All compounds show tautomerism.



Of many types of tauomerism possible, keto-enol tauomerism is the most commonly encountered one.
It is possible in presence of α-C (wrt the carbonyl group), which is the conjugate C-H bond having sp3 hybridization.
Usually it is the α-C of the carbonyl group constituting a triad system, however it could also sometimes be the γ-C, if there is a C=C present on α−β carbons, constituting a quinquad system.
Compound (I) & (II) are have tautomerism in a triad system, while (III) involves a quinquad system as shown here:

