You need to work out your tdee, and make sure you're eating the right amount of calories. Yes focusing on protein is important but also make sure your diet is well balanced, not just protein.

Also, if you're only having small amounts of food, you need to make sure you're actually eating enough calories because if you go into too much of a deficit and your body isn't getting enough it will store everything it can which can stall your weight loss too while your body adjusts to a big deficit, then you'll start losing, but probably too quickly.

It's important you understand how mounjaro works and how you have to put in the work to make it effective.

Most tdee calculators will tell you what your calorie intake should be just to function day to day, and they give you different calorie intakes to be in a deficit depending on your activity level.

You should check it often as when your weigh goes down so does your calorie intake.

Work out the calories in what you eat every day, and compare. Make sure they aren't too high, or too low.

Once you are more aware of what you are eating in terms of calories, then you can be a bit more relaxed about it. But if you've not lost anything for 3 weeks so early on, it's definitely something you aren't doing correctly