Your talking about fluctuations in water retention. To lose 3 pounds of fat requires a calories deficit of 10,500. So even fasting you would have to do enough exercise to burn around 4,000 extra calories each day to do it in 2 days. A marathon burns less than 3,000 calories.

What your weight is on a given day is really only useful in the context of several days to see the trend of highs and lows going up, down or staying about the same.