3 mg is the standard adult dose for melatonin (that is what my pediatrician suggested for my son as well, even though he only weighs 85 pounds), but I've been giving him less so far.

There is also lots of research behind "good sleep hygiene" to help insomnia -- she could have a look at this too. Regular wake up time is really important, and doing something relaxing (not looking at a screen) in the evening also helps.

Don't teenagers generally have a shifted sleep schedule where they aren't sleepy until really late, and want to sleep in most of the morning? Too bad about having to get up for school!