Hi welshy we have 2.5YO triplets, and haven't been abroad with them yet; but our older 2 daughters are very close in age and we have taken them on holiday.
The flight will be fine - it will be the thing you worry about most, but really dont need to! Sticker books are brilliant, mine are all obsessed with colouring so maybe take some colouring bits each. Have they got a DS or anything similar? If not, it might be something you'd consider buying (even just 2nd hand!) before you go as they are brilliant for flying, but also if they're having an 'over-tired' day and just need a break from all the swimming and sun.
Biggest advice would be to assign a DD to each adult. Swap over every day if you want - but anything 'your' child needs, you deal with - anything DHs 'child' needs, then he needs to deal with it. Definitely needs to be done when you're going out in public to the beach/market or even just walking around hotel - its so easy to think 'Oh DH has got both girls', and actually he thinks you've got them! Much easier to be able to concentrate on just one DD each.
Remember to try and get a place close to the kids pool so that you can just sit by the edge and leave them to it - obviously if the kids pool is too deep then you wouldn't do this, but most we've been to the pool is only a Foot or 2 deep so they can wander round by themselves. If you don't already have them, definitely buy DD's those - life jackets for your piece of mind. Obviously you can get them a lot cheaper - but they literally hold all their weight - you won't need to help them at all, even in the deep pool because they have such good buoyancy!
If you plan on spending a lot of time by the kids pool - take some bath toys or be prepared to buy some there so that they have some bits to play with.
All inclusive is brilliant with kids - when I found out I was pregnant with triplets, we decided to go all out on a lovely AI holiday. Our girls loved being able to eat about a million ice creams a day, and its just so much easier!!
Have an amazing time!