Do I have to add my own hydration pack?...
Do I have to add my own hydration pack? Would a camlbak 3L hydration pack fit?
I think you are asking if a hydration reservoir from another brand would fit--and the answer is yes. You can have a 3L Camelbak, Platypus, MSR, or Osprey reservoir in this pack.
However, the Osprey reservoir works great in any pack as it has a framesheet so it can more easily be put in a pack without having to take stuff out of the pack...