Query:

    SELECT s.ip
        , COUNT(DISTINCT s.id)
        , COUNT(DISTINCT i.id) as TotalInputs
        , COUNT(DISTINCT i.input) as UniqInputs
        , COUNT(DISTINCT d.id) as files
        , COUNT(DISTINCT d.shasum) as Uniqfiles 
    FROM sessions s
    LEFT JOIN input i
        ON s.id = i.session
    LEFT JOIN downloads d
        ON s.id = d.session 
    GROUP BY s.ip;

SQL Fiddle

Output:

    ip	| COUNT(DISTINCT s.id)	| TotalInputs	| UniqInputs	| files	| Uniqfiles
    IP2 | 		1				|		3		| 	2 			| 1		| 	1
    IP3 |		3 				| 		5		|  	4 			| 3 	| 	2