Welcome to FT, I'm going to ask a MOD to move this to the DL forum for further comment.

My opinion is that 3 hours is a little on the long side but far from torture on a CRJ-900. The economy seats are 2x2 so one of you will be across an aisle or in a row ahead or behind which shouldn't be an issue with a teenager.

Bring a snack for each of you as the service can be a bit slow on these flights and snacks will be a bit limited in selection.

Beyond that load some music/videos on a tablet or phone & pack a book. To keep the teenager entertained you can buy wifi access for a few bucks.

No reason in my mind to book a connection.