3 weeks is not a plateau.
Weight loss is not linear. You will have times that your weight will go up, times that your weight goes down, and times that your weight will stay the same. Water, food, urine, poo. Hormones, if you are female. All these influence our weight on a daily basis.
6-8 weeks gaining and losing the same 3 pounds was an experience I had. So I celebrated my ability to maintain my weight.
Knowing what you are actually eating will help. This is the part where people give up because they want to see instantaneous and constant change, And that’s just not typical.
First I would ditch the scale in favor of a tape measure. If you’re working out at all you’ll see regular plateaus as you gain muscle.The closer you get to an ideal weight the harder it is to lose. That being said I usually opt for a change in routine. Longer fast. Stop fasting for a few days. Running instead of lifting. This weekend after a horrible night I decided to go mostly alcohol free and due to work schedule I didn’t workout this week and I’ve dropped almost a whole belt hole (to be fair I’m probably properly hydrated). The other thing that could work is a diet change, which you might see if you’re keeping a food journal. Cutting out sugary drinks or removing sugar from your coffee. Eating a salad instead of a sandwich. When I kept a journal I found that I lost more weight eating carbs than trying to remove them (probably more satisfied)