We find a way. Since safety is non-negotiable, we figure out what's the most convenient way to keep our kids safe, seat wise, on that specific trip (will we have tether anchors? How much will we be driving? Etc.) and we work from there. Here's a thread detailing what we did last time:
http://www.car-seat.org/showthread.php?t=106999
Next time we travel, our eldest will probably be old enough to bring a booster (carry on the bottom, pad the top really well and pack in checked luggage) and we can bring the RSTV for the second if we won't be in the car a lot, or the Go if we will (and will have a LATCH position available for her.) So those two will have easy-to-carry on restraints, and we can put them in just the seatbelts on the plane. I'd definitely look into a Safety 1st Go or Ride Safer Travel Vest for your oldest! And possibly easier-to-transport seats for your 2 year old and baby.
Also, we minimize stuff. We pack all entertainment, snacks, diapers etc. that we'll need DURING the flight into one single large diaper bag, so everything is in one bag and that's all we have to carry; then we have one carryon with change of clothes and such that we might need if our luggage is lost into one carryon. So only 2 bags come with us on the plane, and that leaves hands freer. I bring a sling to wear the youngest child, which also helps.