In my opinion I think we don't need a category for gen 3 but 'including all Pokemon' and a separate category for 'evolutions'. As an example, abilities are also available for previous generations since it was introduced in gen 3.
It should be categorized into 'Pokemon generations', in which all gens are belong to this category.
If we make every generation a category, it might be confusing. So ya, 'gen 3' should be merged with other gens. Furthermore, 'evolution' is considerable for it's individual category.