Rate of ${\mathrm{S}}_{\mathrm{N}}1$ reaction is directly dependent on the stability of carbocation intermediate.
(II) is primary alkyl halide.
(I) is secondary alkyl halide.
Primary alkyl halide is less reactive towards ${\mathrm{S}}_{\mathrm{N}}1$ reaction than secondary alkyl halide.
In (III), the carbocation generated is stabilised due to resonance. Hence, (III) is most reactive towards ${\mathrm{S}}_{\mathrm{N}}1$ reaction.