The way I see it, although 898 is a tool of the Foundation, it fits the basic criteria for an SCP in that it is a) Supernatural and/or extranormal in character, and b) may be an ongoing issue for the Foundation. It's comparable to SCP-061, in that sense. Plus, it may become an active project again, someday, if the O5's see fit to start it up in the future.
"three (3) years" may seem arbitrary, but that's just because it really is arbitrary. Nevertheless, it's the format that is used throughout the site (I think). Consistency > Reason.
Exposing only one person at a time was something they did in the early stages of testing, but that's no way to get a large body of data. Complicating matters is the fact that not everyone who received SCP-898 treatment responded to it in the same way. In fact, there were some fairly significant differences between subjects. That's part of the reason why the project was abandoned.