one way would be to Enum.filter to return true for all the fields that are present? and then check that the length or count is larger than 2..

eg

Enum.filter(fields, &present?(changeset, &1)) |> length > 2

or 

Enum.filter(fields, &present?(changeset, &1)) |> Enum.count > 2

I would be very careful to call the validation with the specific fields |> validate_required_inclusion([:title , :content]) - say you add a checkbox in the future and you don’t want code to break or unexpected behaviour as that checkbox all of a sudden counts in this validation..

def validate_required_inclusion(changeset, fields) do
  if Enum.filter(fields, &present?(changeset, &1)) |> length > 2 do
    changeset
  else
    # Add the error to the first field only since Ecto requires a field name for each error.
    add_error(changeset, hd(fields), "Two of these fields must be present: #{inspect fields}")
  end
end

def present?(changeset, field) do
  value = get_field(changeset, field)
  value && value != ""
end