Marseille deserve much more than a three-day visit, if only for the thousands of opportunities that can offer a port city with a great past behind it, however I recommend a visit to the shrine of notre dame de la garde to enjoy the sea breeze and a magnificent view of the city.
Another advice is to take a boat from the old port to the romantic chateau of the fortress and beautiful If iles du Frioul, the main artery of the city is the broad boulevard de la Canebiere which extends towards the east of the vieux port.