you are asking some very knowledgable people for advice here. The advice they are giving you is sound, and very good. If I were you, I would take the advice given. It has never steered me wrong. I come from a zoological/exotic animal background. It is not recommended to house different species together just from a biosecurity standpoint. Chickens and quail can pass diseases between eachother. Size difference is another HUGE issue. Not to mention habitat needs. It isn't fair to either species to try to make one enclosure work for both of them, even if you are talking about housing the quail with only one hen.
Are you getting shipped eggs? From what I have read, shipped eggs don't always have a great hatch rate...you may want to incubate more than three eggs, even if you only want three quail...what species of quail are we talking about? What breed of chicken?
And as for your original question of incubating week old quail eggs, I do not know the answer to that.