You are getting your two relationships mixed up, along with the possibility of not wanting to be tied down yet. So ask yourself this question: what you feel if you knew for sure that Stephen was never going to be an option? Would you still want to be free of Chris?
Now, on this I do not understand: it seems to me you three were practicing poly. Why did it have to end? And if Stephen wasn't satisfied, could not the dynamic have been changed so that he could have been satisfied? Why poly for just a couple of years?
Would Chris consent to polyshipping? Could you not keep Chris and still be free to experience others? Would you be able to allow Chris to see others?