The correct orders of reactivity towards SN1 reaction is 

(1) I > II > III
(2) II > III > I
(3) III > II > I
(4) I > III > II

Dear student,
The answer should be (2) .
This is because, in SN1 reaction the carbocation is formed and is least stable at bridgehead carbon which is present in I.
In III, the carbocation is 10 carbocation which is not stable enough whereas in II, the carbocation formed is 30 which is very much stable. Hence the order of reactivity will follow the order of (2) option.

Hope it helps,
Regards

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