Internal hub is the easiest. Yoy can shift while the bike is not moving, you can have a chain guard, and they don't get out of adj.
If you go vintage 3 speed you want to use a larger cog. They were originally geared so the middle gear was for flats and then a gear for downhill and uphill the uphill gear really wasn't low enough. If you replace the back cog you get a gear for flats and 2 for going uphills. You end up coasting down hill. I think on a newer bike the chain ring is going to be smaller making this modification less necessary.