You might use Enum.chunk_every/2 on a collection.

iex> 1..10 |> Enum.chunk_every(3) |> IO.inspect(charlists: :as_list)
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]

You could also use remainder like this

iex> for x <- 1..10, do: rem x, 3
[1, 2, 0, 1, 2, 0, 1, 2, 0, 1]

def split(list), do: split(list, [], [], [])

defp split([x1, x2, x3 | rest], l1, l2, l3), 
  do: split(rest, [x1 | l1], [x2 | l2], [x3 | l3])
defp split([], l1, l2, l3), 
  do: {Enum.reverse(l1), Enum.reverse(l2), Enum.reverse(l3)}
defp split([x1, x2], l1, l2, l3), 
  do: {Enum.reverse(l1, [x1]), Enum.reverse(l2, [x2]), Enum.reverse(l3)}
defp split([x1], l1, l2, l3), 
  do: {Enum.reverse(l1, [x1]), Enum.reverse(l2), Enum.reverse(l3)}

iex> Test.split(Enum.to_list(1..10))
{[1, 4, 7, 10], [2, 5, 8], [3, 6, 9]}

If your list has always length divisible by 3, you don’t need the last 2 clauses which account for uneven split.

If you don’t care about the order in which they’re put in the bins, this should work. Definitely suboptimal because of the list length and integer division at each step, though.

def chunker(list, parts), do: do_chunk(list, parts, [])

defp do_chunk(_, 0, chunks), do: chunks

defp do_chunk(to_chunk, parts, chunks) do
  chunk_length = to_chunk |> length() |> div(parts)
  {chunk, rest} = Enum.split(to_chunk, chunk_length)
  do_chunk(rest, parts - 1, [chunk | chunks])
end

iex> chunker([1, 2, 3, 4, 5, 6, 7], 4)
[[6, 7], [4, 5], [2, 3], [1]]
iex> chunker([1, 2, 3, 4, 5, 6, 7, 3)
[[5, 6, 7], [3, 4], [1, 2]]
iex> chunker([1, 2, 3, 4, 5, 6, 7, 10)
['\a', [6], [5], [4], [3], [2], [1], [], [], []]