Inception: QUESTIONS QUESTIONS QUESTIONS?

There is only one limbo. Limbo is uncreated dream space, and only things there are stuff from dreams of the past (who happens to be in the same dream trip as you). In this case, only person who's ever been to limbo is Cobbs. In dream level 3, Fisher Jr. is killed by Mal (Cobbs' subconscious), so Cobbs knew where to look for them when they entered into limbo. When Cobbs and Ari enter limbo, they entered using the machine, so they were technically still "living" in dream 3. With a living body in the level right above them, you can be woken up with a kick. That's why Fisher's body in dream three had to be revived with the defibrillator. When Fisher and Ari wake up from limbo, they first wake up in dream level 3, then they escape through the levels. However, when dream level 3 starts to crash, Cobbs' body is "dead," and that's why he washes up on limbo again, this time with the full effects of confusion that limbo brings. That's why Cobbs' seemed so confused at the beginning of the movie when he first meets Saito.