snippet.ex

stock_list = [["Mint", "unavailable", "UK 4"], ["Nude", "unavailable", "UK 4"],
             ["Mint", "unavailable", "UK 6"], ["Nude", "unavailable", "UK 6"],
             ["Mint", "unavailable", "UK 8"], ["Nude", "low_stock", "UK 8"],
             ["Mint", "unavailable", "UK 10"], ["Nude", "unavailable", "UK 10"],
             ["Mint", "unavailable", "UK 12"], ["Nude", "low_stock", "UK 12"],
             ["Mint", "unavailable", "UK 14"], ["Nude", "low_stock", "UK 14"],
             ["Mint", "unavailable", "UK 16"], ["Nude", "low_stock", "UK 16"],
             ["Mint", "unavailable", "UK 18"], ["Nude", "available", "UK 18"]]

into

%{ "Mint" => %{"unavailable" => ["UK 18", "UK 16", "UK 14", "UK 12", "UK 10",
    "UK 8", "UK 6", "UK 4"]},
   "Nude" => %{"available" => ["UK 18"],
   "low_stock" => ["UK 16", "UK 14", "UK 12", "UK 8"],
   "unavailable" => ["UK 10", "UK 6", "UK 4"]}
}

what you need

stock_list
|> Enum.reduce(%{}, fn [name, stock, type], acc ->
  Map.update(acc, name, %{stock => [type]}, fn stock_map -> Map.update(stock_map, stock, [type], &[type | &1]) end)
end)