Yes, it's perfectly normal to gain 3 lbs over the course of a day. That's why I try to dissuade myself from stepping back on the scale after my morning weigh-in.
The scale weighs fat; it also weighs muscle, bone, water, stomach contents, and waste. Bone stays constant, muscle doesn't change much, and fat actually takes a few days to add or subtract (remember 3500 calories/pound of fat). So if you're eating at a calorie deficit, you WILL lose fat. The scale may just be showing the weight of the food in your stomach, and the liquid you drank, and the waste in your intestines still working its way through... but under all that, the fat is slowly working off. Eventually it WILL show up as a lower number on the scale.
~~ Synger ~~