It's impossible to say how much of your weight gain is water weight and will go away. You'll just have to stick it out and see!
If you really went overboard with food, then I'm sad to say it is possible there is some weight gain there. Probably not the whole 3 pounds, though!
Have you got a calorie target, or some other food target, and are you sticking with it, come what may? If so, then don't worry--it will work!