took it to doesnt think its the cat.

Pull the plug out and look at it, compare it to cyl 2 the swap the plugs between 2 and 3. Did the miss follow the plug? Find another wire the about the same length as #3 and swap them does the miss follow or go away? You could move the injector to another cyl. although it is unlikely to cause this problem.

I think its the rear o2 sensor.

Also an o2 won't cause a miss on one cyl. But a miss will mess up the o2 readings.