Why does your weight fluctuate 3 -5 pounds each day?

All kinds of reasons. There are the obvious reasons like food, water, excrement, etc. For example, a half litre bottle of water weighs one pound, and most foods are roughyl the same density as water, so eating a meal and drinking a glass of water can add a few pounds. Also, using the toilets can drop a pound or two.

Also, the placement of the moon in the sky has an effect on weight. When the moon is overhead, you weigh less. Remember, this is independent of the time of day. The moon is "out" a lot during the day when you can't see it, so your weight fluctuates with that as well.

To get a good measure of your weight, weigh yourself a couple times a day for a week and take an average.