Four of us arriving by car, late June, and staying in Cassis for three nights. I heard it was beautiful. Any ideas?

It's is beautiful, but there are very few outstanding properties inside Cassis itself. It will all depend on what you're looking for and your budget. The last time we rented an apartment overlooking the bay. You can try L'Hotel du Grand Jardin and the Hotel le Cassiden, both near the village center.

Les Roches Blanches is a nice stroll to center and much nicer than staying in the center. They have rooms facing the water and it is a very nice hotel and close enough to town but in a nice quiet area. Cassis is beautiful.

Is a car required to stay there? Can one get there by train if one does not want to drive?
I have always wanted to go back. In 1997, we joined a small group of friends with a leader who is French. We spent a day in Cassis before moving on southward. My husband and I really liked Cassis and talked about spending some days there.

3 days in Cassis is a bit long IMHO. Nice for a look see kind of day; a boat ride to calanques perhaps. Driving on the corniche road is nice, but requires a car.

Of course you can get there by train, a lot of people go for a day trip (myself included) from Marseille. I took a shuttle bus from the train station to main part of town near the boats, but it would only be a very short taxi ride. And you wouldn't need a car while in Cassis if you didn't wnt to go anywhere else.

Cassis is really small so there can't be that many choices of hotels. The tourisme office website seems to have a comprehensive list, make sure you note the real location of the hotel http://www.ot-cassis.com/en/hotels.html