In both the anime and manga the Sanbi (three-tails) did originally have a jinchuuriki, but somehow, we still have no idea how, the bijuu and its host were seperated, and thus it was hostless when captured by Deidara and Tobi. According to the data book, the name of Sanbi's host (prior to their seperation) was Yagura. Yagura was also the name of the identical Yondaime Mizukage. That is no coincidence, they were the same people.
In fact, there's probably a few topics about this on a page or two back, when the chapter was first released. This isn't something that hasn't caught our attention before, but it's certainly an interesting topic; this lead to theories, when it was claimed that Yagura may have been being manipulated, that it was likely Uchiha Madara doing so (through bijuu control?).
