Does the osprey atmos 25 take a 3 litre hydration bladder or only a 2 litre?

It can take 2 hydration bladders. I think it is 2 litres.

I'm pretty sure I was using a three-litre Source bladder in mine, leastways that's what I normally use and I don't remember not being able to fit one in.

There's also a supplementary bladder pocket between the mesh and the back of the sac as you probably know.