Solution: https://www.reddit.com/r/dailyprogrammer/comments/6melen/20170710_challenge_323_easy_3sum/

ThreeTwentyThree.ex

defmodule ThreeTwentyThree do
  def find_sum_zero_triplets(list) do
    for x <- list, y <- list, z <- list, filter(list, x,y,z) do
      Enum.sort([x,y,z])
    end
    |> Enum.uniq
  end

  defp filter(list, x,y,z) do
    ([x,y,z] -- list == []) && (x+y+z == 0)
  end

  def print(result) do
    IO.inspect(result, label: "Count: #{length(result)}")
  end
end

# -----------------------------------------------------------------------------

test_data = [
  [4, 5, -1, -2, -7, 2, -5, -3, -7, -3, 1],
  [-1, -6, -3, -7, 5, -8, 2, -8, 1],
  [-5, -1, -4, 2, 9, -9, -6, -1, -7],
]

# print the results

test_data
|> Enum.map(&ThreeTwentyThree.find_sum_zero_triplets/1)
|> Enum.map(&ThreeTwentyThree.print/1)