Stability order of these benzyl carbocations is:

Among the given options, all carbocations are resonance stabilized. Therefore, the comparison done on the basis of hyperconjugation. In carbocation (I), there are three alpha-hydrogens, hence it is the most stable. In carbocation (II), there are two alpha-hydrogens, hence it is more stable than carbocation (III); that has one alpha hydrogen atom.

 I > II > III