Sounds like an adventure! We have two kids close in age, and at about three, they were each ready to start some of their own hiking. You might consider having two carriers — one for the baby and on for the twins to switch in and out of.
Scavenger hunt, "I spy," and leaf rubbings in notebooks are all fun activities to keep the kiddos occupied, too.
As for packs, I had an old REI pack that I loved and finally gave out. I replaced it with the Gregory Sketch 28 that I can use for work gear and on family hikes. My wife has a smaller 18L Gregory that she loves for day hikes. I personally like having something with at least 28L for myself because I end up carrying extra water bottles, layers, etc.
Hope that helps!