Don't you want much smaller summary table?

WITH everything AS(
  SELECT maker, 
    COUNT(DISTINCT pc.model) pc, 
    COUNT(DISTINCT l.model) l, 
    COUNT(DISTINCT pr.model) pr
  FROM Product p 
  LEFT JOIN Pc pc ON p.model = pc.model
  LEFT JOIN Laptop l ON p.model = l.model
  LEFT JOIN Printer pr ON p.model = pr.model
  GROUP BY maker
)
SELECT maker, pc/(pc+l+pr) pc, l/(pc+l+pr) l, pr/(pc+l+pr) pr
WHERE (pc+l+pr)>0