sum3 :: Num a => [a] -> [a] -> [a] -> [a]
sum3 xs ys zs = sum3Helper [] 0
  where
    maxn = max (max (length xs) (length ys)) (length zs)
    sum3Helper res curn | curn == maxn = reverse res
                        | otherwise = sum3Helper ((getElem xs curn 0 + getElem ys curn 0 + getElem zs curn 0) : res) (curn + 1)

    getElem [] n curn = 0
    getElem (x:xs) n curn | curn == n = x
                          | otherwise = getElem xs n (curn + 1)