Lessons learned from my travels...
For the baby, I would ask for seats with a bassinet if your airline has them. Makes things a lot easier! Make sure you book seats together, if it's a plane that doesn't have 4 seats in a row think about how you want to sit.
Also on board ask if they have an activity pack for your 3 year old.
It can be a lot of hassle to take baby food through security. You can also buy it from a shop on the other side.
Book a child meal for your 3 year old.
Do a bit of research in advance about where the kids play areas are in the airports you're using. Really helps when you're waiting for planes.
Get to airports with plenty of time to spare.
Take lots of toys and books, including a couple new ones.
Take a few snacks.
Most usefully, tell your 3 yr old all about it in advance. Tell them about security, about the plane, about the seatbelt and how they need to keep it when the light is on, about the meals on the plane, about not kicking the set in front, about all the other people on the plane.