At present, two fixtures on one switch are in parallel; so a three wire cable connects the two fixtures. How to convert existing three wire cable to support two switches, one for each fixture? Reason is that the three wire cable is buried in finished ceiling, and running another cable to one of the fixtures would be very awkward. Fixtures and switch on wall by doorway have joists running crosswise between, so getting a snake through would be difficult. If not advisable to use three wire cable for two circuits, what is problem or risk?"

By "three wire" do you mean black, white and ground(bare)?
If so then you CANNOT do what you intend. The bare wire cannot be used as a circuit conductor.