I have no problem with a 3 hour movie, the longer the merrier. I'm also not suggesting them to cut the movie in less than 3 hours just to make room for more showings. So thank you because I don't have that kind of mentality, for the record.
But yes, it could affect ticket sales and knowing FOX's track record (most of their movies are shorter than 120 mins), they would want to get more money by booking more showings in theaters. And like the previous X-Men movies, there will be scenes that won't make the final cut.