I have to respectfully disagree.
For one, the material you speak of has nowhere near the recognition, power, and interest that that which lies within the main body of The Lord of the Rings or The Hobbit commands. Second, I don't think PJ has as much carte blanche with the studios to do whatever he wants as you might think. The Hobbit movies did not generate the same revenue or critical acclaim that LOTR did, and to my opinion (in which I am joined by many,) failed to recapture a lot of the magic of the movies of 15 years ago. For a major studio to take a gamble on such a project, the prospective audience would need a little more awareness of the subject matter and plot than to merely say it happens to set in Middle-earth and just hope people go see it on that merit alone.
Sure the hard core fans would line up, but the casual book fans, and those that are maybe only aware of the main titles by name, would not. The only thing that would even come close in terms of recognition would be the Silmarillion. Ask people to go see a movie about Castamir the Usurper and Umbardacil and people will go "The what, with the who?" Like it or not, they are footnotes in the history of Middle-earth, and it's not the stuff that would sustain three action-packed blockbuster movies like you propose.
On TV however, is where such a story could shine. I could see a cable network picking up a story like this as a miniseries or even something that would run over a couple of seasons. With the precedent of Game of Thrones, a political-fantasy story, filled with intrigue, charismatic characters, and beautiful settings, could really take off. I get excited just thinking of what Umbar would look like onscreen.