As John said, it is unlikely that you put on 3 pounds of FAT in 2 days of less-than stellar dietary choices -- unless you were really packing it in.
If we assume your BMR is say 1,500 calories. (Just a random number) that means that you burn 1,500 calories a day just existing. That doesn't even factor in calories burned by walking, talking, exercising in the gym, doing dishes, etc. Add all of that up, and you get your Total Daily Energy Expenditure (TDEE). For arguements sake, lets say that number is 2,500 calories.
Knowing that it takes approximately 3,500 calories to gain a pound of fat, you would have had to have eaten (as John mentioned) 10,500 calories IN ADDITION TO your TDEE of 2500 calories in the course of two days. So, for this example, unless you ate 15,500 calories during that time, you did not gain 3 pounds of fat.
I suggest you read the sticky on water weight fluctuation... it will help to put your mind at ease.