This query:

• Uses ANSI JOIN (LEFT, INNER, ...)
• Uses LEFT JOIN for each table in order to count Sessions IP without Input or Downloads
• Uses DISTINCT for each COUNT in order to remove duplicates added by the JOIN between tables
• Counts values for totals and ids for unique counts

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

This query:

• Uses ANSI JOIN (LEFT, INNER, ...)
• Uses LEFT JOIN for each table in order to count Sessions IPs without Input or Downloads
• Uses DISTINCT for each COUNT in order to remove duplicates added by the JOIN between tables
• Counts values for totals and counts ids for unique counts

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

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

This query:

• Uses ANSI JOIN (LEFT, INNER, ...)
• Uses LEFT JOIN for each table in order to count Sessions IP without Input or Downloads
• Uses DISTINCT for each COUNT in order to remove duplicates added by the JOIN between tables
• Counts values for totals and ids for unique counts

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