Is 124.2 mg/dl a normal blood sugar level for a 17 years old girl after a meal for after 3 hours?

Probably normal. Hi. There aren't diagnostic criteria for normal sugar 3 hours after a mixed meal that I'm aware of, but I'd say that seems pretty much normal. Variables would be how much carb and fat were in the mixed meal; they influence how much and how quickly sugar will appear in the blood. If you're worried, talk with your doc about doing a 75-gram oral glucose tolerance test. Good luck!
Well... Yes it could be normal, depending on what you ate before, and the method used to measure the blood sugar. Most accurate test is a fasting blood glucose done by a laboratory.