It might seem odd for you to be able to build muscle mass and shrink your body (be leaner) as your body fat decreases. Muscle mass takes up less space than body fat. That’s how a person can lose 10 pounds of fat, replace it with muscle mass, still weigh the same and have a leaner (and smaller) body.
So, its not wise to gauge your success solely by the weigh scale. After 3 weeks on a strength and cardio exercise program, you could weigh the same (or even a little more) and have significantly less body fat. Its important to check your body fat every week to see if your body is shrinking. Lost inches and body fat should be the main focus of your exercise program. This will speed up your metabolism and the weight will begin to melt away and stay away.
You must build and maintain your muscles or they will shrink (use it or lose it). Your muscles will shrink (atrophy) if you don't continue to do strength training. Muscle does not ever change into fat. Fat cells will shrink but they will fill up again when you are inactive for a long period of time.
Having said all that, you will gain weight and fat if you maintain a daily caloric surplus. A caloric surplus happens when you eat more calories than you burn. It doesn’t matter how hard you workout. A caloric surplus will lead you to the fat farm every time! So, eat enough to build muscle and burn fat. That usually means maintaining a daily caloric deficit of about 300 to 600 calories. If you have one cheat day a week and have a small caloric surplus, that should not derail your fat loss efforts. Just continue to do your regular workouts and get back on track the next day.
Start a meal plan that you can maintain. Severe calorie restriction diets cannot be maintained. When you have rapid weight loss through severe calorie restriction, much of that weight loss will be water weight (and some muscle mass). So, you find yourself regaining this weight quickly when you begin to eat the carbohydrates, proteins and fats that your body needs. Why? Because you will also replenish water stores in your body's cells.
Lose as little water weight as possible so your body is forced to burn more body fat.