Weight gain/working out

[KMO7970]

Echoing everyone else, yes, this is likely water weight. Three weeks is not enough time to truly gain muscle. My advice is to stay off the scale. Focus on how you feel and how your clothes fit. When I started working out many years ago, I noticed that, as I added resistance training, that I had gained 10 pounds but dropped 2 pants sizes! You'll get to where you want to be fitness-wise. Just keep doing what you're doing!