First off, you're probably not doing anything wrong. Don't get discouraged.
Forget the scale for a moment
The scale is the worst way to track progress. If you're training and eating well, you're losing fat and building muscle at the same time, and muscle is denser than fat. The number can sit still while your body changes underneath.
Better checks: how are your clothes fitting? What do your progress photos show? A check-in every 2 to 4 weeks tells you more than a daily weigh-in.
Three weeks is still early. This is exactly where most people quit, right before the results show.
Then check the basics
Real, whole food. Most meals built from simple, minimally processed food.
Protein at every meal. Meat, eggs, fish, chicken, dairy.
Your calorie target as a guide. You don't need to be perfect, but the overall picture has to match your goal.
The extras. Oils, butter, sauces, dressings, snacks, drinks and bites while cooking add up.
Your program. Resistance training and progressing over time, not more cardio.
Daily movement. Walking and general activity still matter outside your workouts.
Don't overcorrect
You usually don't need to cut carbs, avoid fats, start fasting, add cardio, change programs or eat as little as possible. Keep it simple and give the plan time.
Truly stuck?
If you've been consistent for more than three weeks and your measurements and photos aren't moving either: check your portions honestly, get your steps up, and look at your sleep before touching your calories. One small change at a time.