How can I prevent gaining it all back?

You can get a grip, and think long term. You lost water weight sport, not a lot more. Gaining it back won't change a thing. Exercise is the key to losing and maintaining weight, your diet should just compliment it.Keep the exercise enjoyable, again think long term, it must become a habit first, that takes time, if exercise or diet is painful for your brain and/or body, you'll quit. Your freaking about 3 pounds=brain pain, so chill, be sensible your not just a walking, talking, calorie bank, your a human being, go for great health, and you'll get what you want and so much more! ( and it will be easier then you think, enjoyable and sensible!)