Presumably, nothing happens with three and under. At the same time, those other three year-olds are going to have parents, teachers, and aren't going to stay three forever. All it would take would be one slip with the paperwork, and then you've got the four year old who slipped in killing everybody else in the room (remember, you're not going to be able to send anyone in to stop them without a hazmat suit, and any observance would have to be done from a remote location) before trying to kill 053 and dieing. Heck, without testing, we don't even know at what point over three starts the bloodlust. For all we know, it may be three and a half, three and a quarter, or even one day over exactly three years old. And no, I don't think even the Foundation would give permission to keep shoving three year olds in there until we establish what age is the cut off. If there was some way to exploit it, maybe, but with no possible benefit? They're not going to do that just to satisfy curiosity.
Now that I think about it, I've got to wonder how exactly that three and younger safeline was established. My best guess is that it has something to do with the circumstances of her discovery, but… I'm not sure. Either that, or something involving an incident that it better left unmentioned.
So yeah, I know the best thing is to keep her locked up for the rest of her natural life. If she even has a natural life. It doesn't mention how long we've had her, so it's possible she's not aging, or doing so slowly. I think personally, the scariest would be if she was aging normally, and we just haven't had her long. If she stays age three, it's almost easier to justify and, for me, live with locking her in a cage. If she's going to grow up, and become an adult, all having interacted with nothing more than hazmat suits and the four walls around her… That's a very sad thought.