Since $${S_N}1$$ reactions involve the formation of carbocation as intermediate in the rate determining step, more is the stability of carbocation higher will be reactivity of alkyl halides towards $${S_N}1$$ route. Now we know that stability of carbocations follows the order : $${3^ \circ } > {2^ \circ } > {1^ \circ },\,\,\,$$ $${S_N}1$$ reactivity should also follow the same order. $${3^ \circ } > {2^ \circ } > 1 > $$ Methyl ($${{S_N}1}$$ reactivity)