SQL 4

SELECT modell
FROM (SELECT modell,ar FROM `$t_pck`
        UNION
        SELECT modell,ar FROM `$t_laptopok`
        UNION
        SELECT modell,ar FROM `$t_nyomtatok`
        ORDER BY ar DESC) t
LIMIT 1;

SELECT gyarto FROM `$t_termekek`
JOIN `$t_nyomtatok` USING(`modell`)
WHERE `szines` = 'igen'
ORDER BY ar ASC
LIMIT 1;

SELECT gyarto FROM `$t_pck`
JOIN `$t_termekek` USING (`modell`)
WHERE sebesseg > (SELECT AVG(sebesseg) FROM `$t_pck`
                        WHERE memoria < (SELECT AVG(memoria)
                                                FROM `$t_pck`))
                        AND memoria < (SELECT AVG(memoria)
                        FROM `$t_pck`)
GROUP BY `gyarto`;