Weight is more than fat.
Weight includes... water retention, hormonal nonsense, the food that is literally right this moment traveling through our systems, but has not been turned into fat or energy to burn.
Being up 3 pounds doesn't really tell you anything. Yes, you probably ate at a higher calorie level, but that likely means you ate more food... and therefore the weight of that food is going through your intestines. Not only that, but indulging in more food can often mean more water retention.
3 pounds is about equal to 10,500 calories. Did you eat over your calories by 10,000? If so, then maybe you gained fat.
If not... which is more likely, then it's water, food and etc just sticking around on your body.
You might also want to read this sticky: Fluctuations in Scale Weight and Water Weight