I just don't see the point in a remaster of FC3; FC4 surpasses it in every meaningful way and is available right now on PS4/One.
I've only played maybe a few hours of Far Cry 4 but for some reason I don't like it quite as much as 3. The reason is really complicated and subjective though.
Firstly, I still prefer Far Cry 2 to both of them. I recognize FC2's execution was way more flawed and it had huge problems, but I like that it's a less structured and more open-ended game than the later ones. You can kind of see a slide from open-ended immersive sim to somewhat linear "gamey game" from 2-to-4 (I haven't played Primal). 3 added the towers and made the main story a lot more linear but in between that just enough of the immersive sim was still there for that feeling to emerge from time to time. The best moments of FC3 for me are having random stuff happen in the jungle and making my own way to clearing outposts. FC4 took the base clearing and ran with it, but I feel like Ubisoft made that part much more gamey. It sort of turned the bases into trials. The random events happening all over the place also somehow feel less organic in FC4 and more, well, random. Add to that the increased collectibles, towers, and still linear main missions.
I don't even know if I'm going to finish FC4 because Zelda is pretty much already providing everything I wanted from that game. I even hesitated for a long time before getting FC4 (for like $10) after I had Metal Gear Solid V, which also provided just about everything I wanted out of Far Cry.
Basically the next main Far Cry needs to look at MGSV and BOTW and try to make the actual next-gen successor to those games.