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

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.

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!