MySQL:seleccione 3 tablas con la sintaxis de combinación izquierda correcta

pruébalo (sin usar subconsulta )

SELECT  a.name, b.cars, count(c.id) as  number
FROM    users a 
            INNER JOIN data_tmp b
                on a.who = b.who
            LEFT JOIN data c
                on a.who = c.who AND
                   a.name = c.name
WHERE    a.who = 2 
GROUP BY a.name, b.cars

esto funciona en diferentes servidores:

SERVIDOR MSSQL @ SQLFIDDLE
MYSQL @ SQLFIDDLE