My advice would be to choose any one of the stories (it doesn't matter so much which one, because if it did matter you would know which one, and there would be no dilemma) and work on it exclusively for a while. If, after a couple of weeks, that kind of total immersion hasn't pulled you into the story in a whole new way, put it aside and go on to one of the others. Repeat if necessary.
I do know writers who are able to work on more than one project at once. I, however, am not one of them. I need to live with my characters, eat, sleep, and go about my daily activities with those characters always in my head. When I don't do that, the writing suffers.
If someone who works on more than one story at a time is consistently turning out excellent and satisfying work, then I would be the last person to suggest that they do anything different. But if you are dissatisfied with your progress as a writer, or if you never seem to finish anything, it might be because you are scattering your energies.