Mostly likely it's water weight, retained water. If you aren't drinking enough water (min of 64oz daily) or are eating too much salt this may cause the gain you're experiencing. Weight also naturally fluctuates some, so don't fret. This is very common & nothing to worry about. As metioned before, TOM can also play a key role in this experience. If you haven't cheated it's virtually impossible to gain 3lbs so quickly, unless it's water. Stick with what you're doing, up your water intake, watch your salt intake, & this should pass in a few days. Good luck.
God Bless,
Amy-Lynne