Ventilating an un-insulated attic

Is it necessary to ventilate the un-insulated attic of a three-season cottage?

Necessary? No. And possibly pointless: Venting your attic is not going to make a big difference in temperature. Attics will still be hot even if you ventilate because the sun is beating down on the roof. Venting will cool the attic a bit and may extend shingle life by a year or two, but choosing a lighter-coloured shingle that reflects the sun is more effective. And insulating the floor of the attic will keep the heat from getting into the cottage below. If you do decide to insulate the floor, you should definitely ventilate the roof too, otherwise the heat can’t escape and will build up, leading to high humidity and possible future moisture problems.