Yes, one can use min_key: {__MODULE__, :some_key}, max_key: {__MODULE__, :some_key, 0, 0}, using exclusive ranges. Note that this is not perfect, as it would for example include a tuple like {__MODULE__, "xxx"} or {__MODULE__, :some_key, 0, -1}.
That said, for practical cases it might be specific enough, when you know there won’t be any 2- or 4-tuple key of that kind. You can also use a filter function on top of min/max key to filter out anything that is not a 3-tuple.
If you have more limits on the third element, one can do better than that, leveraging on the term ordering.
What would be really useful would be to have a “minus infinite”. The nice thing about it, is that it would be smaller than any term. That way, one could change the above to min_key: {__MODULE__, :some_key, minus_infinite}, max_key: {__MODULE__, :some_key, 0, minus_infinite}, and match only the tuples you describe. As far as I know though, there is nothing like that in Erlang/Elixir.
Your observation also gives me the chance to briefly explain why that’s the case.
