DD11 uses headphones with her Nintendo DS becasue there is alot of "white static" on planes.
The ped. told me to give DS3 (flew once at 11months, 2yrs, 2.5 yrs). Benedryl a 1/2 hr before take off the first time we flew. It was a miracle! Now he gets a dose once we are on the plane and settled everytime we fly.
Just an FYI- Since he is so little he has to have the liquid. I have never had a problem getting it through security (at DTW-Detroit metro, or at FNT-Flint). I always carry it in a little bag (a 2 bottle holding bag) and open it and say it is for DS, to keep him calm on the flight. You may want your ped to write a script or a note (with his info on it) saying it what it was prescribed for. I did have another prescription once, so it was obvious he needed it.
I have always had the big bottle, but with the new individual dose packets that might work for you. To much in the prepackaged dose for a 3 y.o.