What exactly is water weight? I wonder if that is the 3# I lost. ???

For example, when I eat pizza, my weight will increase the next day because pizza has too much sodium in it. It means my body is retaining water.

Anytime, you eat foods loaded with sodium/salt, your body retains the water. So anytime, my weight goes up really quickly, that's usually a good indication I'm retaining water. But if you only lost 3 pounds for the very first time, this may very well be fat weight. Check in again a day or 2 later, and you should know whether it was water weight or not. But 3 pounds is quite feasible.