Unlike my past theories, i dug deep into this. and i mean DEEP. so let's go through the trailer.
scene1: a hallway. nothing to exciting.
scene1.5: He will come back... i know who that is :)
scene2: more hallway :/
scene 2.5: he always does... so it has to be some one from the past 2 games.
scene3: now this one is interesting, Bonnie, Chica and Freddy. Bonnie looks at you later in this scene.
scene3.5: we have a place for him... interesting...
scene4: that is the twitchiest animatronic i have EVER seen.
that's as far as i'll go, because that's all the trailer has to offer to this theory.
watch the full trailer here: Fnaf 3 trailer
now let's look at the story: Thirty years after Freddy Fazbear's Pizza closed it's doors, the events that took place there have become nothing more than a rumor and a childhood memory, but the owners of "Fazbear's Fright: The Horror Attraction" are determined to revive the legend and make the experience as authentic as possible for patrons, going to great lengths to find anything that might have survived decades of neglect and ruin. 

At first there were only empty shells, a hand, a hook, an old paper-plate doll, but then a remarkable discovery was made... 

The attraction now has one animatronic

Now here is my theory: that animatronic that survived 30 years of ruin, that animatronic that is rotten and destroyed, is none other than Fnaf1's creepiest animatronic, Bonnie the bunny.

here's what the evidence is: the secene with the three animatronics is a flashback. how do i know this? well, in one of the screenshots, you see freddy's head on the wall. but freddy's here to. and his head. in fact, all the animatronics in that scene are in the excact repair as they were in the first game. they wouldn't be like that after 30 years, would they? no. but why does bonnie look at you? because he knows. here's another piece of evidence i found: that mysteorious animatronic is obviously a bunny of some sort. remember the texts saying he will come back, he always does. well bonnie, as far as we know, has appeared in every game now. remember that teaser sayin "only one left"? it's not saying fnaf3 is the last game, it's saying that there is only one animatonic that still works left. in the story, the remarkable discovery is that there is an animatronic that survived 30 years of neglect. bonnie. those purple words saying "he always does" refers to not the purple man, but the purple bunny. still not convinced? well then here is the last piece of evidence: we see scropped parts of every animatronic that could fit this mystierious animatronic except Bonnie. you can see golden freddy's suit right outside your door. now i can assure you that bonnie is in fact, the "only one left". anyone think that they can disprove this theory, let me know in the comments (unless you say that fanf2 is a sequal, because it's not). alright, good bye!