When putting a new shingles on top of old ones ( I know you can only go three deep) do I need to put tar paper over top on old shingles before putting new ones down?

No tar paper but you will need to check when you pull your permit. Some towns allow 2 layers, some 3, mine only one.

Not adding a new roof just replacing shingles. Do I still need a permit. Live in Maryland

Probably not but best to ask your local building inspector.