Starter feed is fine for the whole flock, indefinitely. The main differences are protein and calcium. The extra calcium in laer is harmful to young chicks. 3 month may be old enough to eat layer, although personally I'd wait at least 2 more months, or when they start laying, to be safe, and also to give the chicks the bit of extra protein. Probably the biggest problem with feeding adults starter feed is they don't need that much protein so it is a bit of a waste of money. Of course if you feed the whole flock starter, the laying hens will need a source of calcium. Baby chicks do not usually bother oyster shell offered separately. Many people feed the whole flock a flock raiser or grower feed routinely. Many a rooster has lived for years on layer feed, but they don't need that extra calcium, either.