S.T.
dont' be afriad to use a harness & "leash" on your 3 yr old. I use done in Wash DC when my son was almost 3 - he figured out too quickly how to take it off - but until that time it kept him close. Some people will make faces or snarky comments - but my feeling is that I'd rather make sure my child is safe than worry aout rude comments.
Bring new toys for the plane - things they've not seen beofre. Color magic markers and paper? magnetic play things and a small metal tray ( like a toaster oven tray)? Small action figures that they like. Then hone up on games like "I spy with my little eye...." (I'm sure you can google more kinds liek that) which will often keep them engaged since they get to interact with you. Then as a last resort bring some cookies, pretzels, chocolate or other treats to make it through those final moments on the plane. Ave them for when all other options are exhausted. Also consider investing in a portable DVD player with headphones - and a new movie or two. Works like a charm most of the time.