Re: First Class 3 or X-Men 5?
Things need to progress into the future. I don't think there should be another film solely dedicated to the past. That said, I see no reason why a following film couldn't incorporate both times as this one is doing.
My pick? Use Sinister as the villain across the timelines, with Charles recruiting Scott and Jean in the 80's and the final battle between the X-Men and Sinister taking place in the present day. All this could lead into Apocalypse for the next installment.