I assume you have a double stroller with that age spread. Take the double, and a bjorn/sling/backpack for the baby so both older kids can ride if they need to. Mine aren't quite so close in age, but when I flew to Jamaica by myself with my 2 oldest at ages 3 and 1, my double stroller was ESSENTIAL. Children who are strapped in can't get lost, hurt or destroy things while you are trying to concentrate on checking in, spotting luggage or waiting in line in customs. A leash thing is a good thing to have too.
Play doh, pipe cleaners, stickers, tape and paper make great airplane entertainment. A pillow case makes a great changing table cover that takes up less space in your bag, and covers enough area to change your 2 year old on. I always had enough diapers for my 2 year old so if my 4 year old was desperate to pee during turbulence or final approach to landing (when flight attendants get snippy and you don't want to be unbuckled for any longer than absolutely necessary)I could slip the diaper on her, let her pee in it, then pull it right off and wrap it up. Produce bags make great trash bags for sticky snacks and diapers. For boys, an empty water bottle makes a perfect emergency urinal.
Have a great trip!
S.,
SAHM of 3 (9, 6 1/2, and 2 1/2)