We are going on vacation soon and trying to figure out the logistics. Why is this so hard?

We have 3 kids, all in car seats. I do not want to check the car seats so if we bring them we will need to bring them through the airport with us.

We have 2 Diono Radians that we can get straps to carry like a back pack. Not ideal, but I guess it's fine. What do we do about a third car seat? We considered one of those car seat harness things but my oldest still slumps forward when he falls asleep in the car so it makes me nervous. I know you can rent them but I really hesitate to do that, with unknown history. Am I missing something totally obvious? We have never flown with 3 kids before so we are clueless! Help, please!