This rumor started out of a response that E L James gave on twitter about the possibility of a 4th book in the series. When asked if there would be another book, she said, “never say never.” This isn’t the first, and probably not the last time she will say that. That has been her response to that question for as long as I can remember.
In more in-depth answers to the question she has said she is working on another series (she’s drafted the first of 2 books as of a few months ago, but has been busy with filming since) and indicated she wanted to finish that book series before working on another project. Whether that other project is a 4th Fifty Shades of Grey book, remains to be seen. My feeling is that, unless it’s something she does in conjunction with the film I’m not holding my breath for a 4th book any time soon.
What I do think we will see is that she’ll continue to do an occasional outtake about Christian and Ana for the major anniversaries, like she has done in the past. She’ll continue to say “never say never” and if she actually does do it, we’ll find out like a week before from the horses mouth (pun intended).