It also looks like each clan member has 2 attacks, but there is no limit as to the number of times you can attack each person. But only the best attack (as defined by the most stars gained) counts towards the star total.
So once a base is 3-starred it doesn't make sense to attack them anymore, as it won't contribute to the star total.
ETA: The most stars you can get from a base is 3. So if 3 attacks all one star a base, that is 3 stars towards the clan total. But if 1 attack 3 stars a base, that is also 3 stars towards the clan total. In either case, once 3 stars are earned towards the clan total, it no longer makes sense to attack that base as 3 stars is the max you can get from any one base.