If it's a night flight try to feed them at the airport so you're not waiting for food for them to be delivered by the crew & they're not getting tired & hungry waiting. Try to wear them out at the airport, let them run off steam & you could try to stay at the gate for as long as poss so you aren't cooped up on the plane for longer than necessary. Try to keep the youngest awake as much as poss (but don't get her over tired!)
Get them into pj's asap (depending on flight time) so either as soon as you get on the plane or maybe easier to do it before you board with 2 of them & see if you can get them to sleep asap. Tell the eldest she can be a 'big girl' & stay up a bit later than her sister & watch tv/dvd while you settle the baby & take a sealabe cup so she can get some milk or just drink in general. Even give them some calpol to make them sleepy (take sachets so they don't confiscate them).
If your 3.5 year old isn't in nappies, consider putting her in 1 (pull up even) cos nothing worse than little one who needs loo when there's turbulence & might make it easier for you than having to go to loo with her when you have another to look after (although I'm sure the crew would help out)
Can you get/borrow a trunki for the 3.5 year old. They are good & she can zoom around on it herself & occupy her if getting bored at the airport & in the queues. If you go for the trunki it can have all her stuff in & be her big girl hand luggage.
Have you thought about a sling/carrier for the baby? Might be easier than having to manoevre hand luggage/buggy/3.5 year old or if you did still take the buggy, you could always have baby in sling & 3.5 year old in buggy around the airport or to/from plane if she's tired or you need to get somewhere fast. If you don't use a sling & just take a buggy you should be able to stick some luggage on the buggy to carry & you could always try to get a rucksack for your hand lugagge to make it easier to carry.
DVD player great idea (or laptop) with fave dvds. Take plenty new stuff to keep occupied (if they decide not to sleep!). Stickers are great, small books, colouring stuff, fave teddy, some cheap stuff from asda, pound shop so if it goes missing it doesn't matter. Take plenty snacks & spare bottles for baby & take more milk (& nappies) than you think you need (& baby food in pouches are good)
If they have smallish pillows or blankets take them.
Make sure you have a change of clothes or 2 for the kids & at least a spare tshirt or something for you. Also crocs are good for the eldest if she needs the loo during the flight, easier to get on/off than shoes for the lovely aircraft loos
. Ziplock bags are good to put smelly/dirty clothes in.
Don't be frightened to ask the crew or even pleasant passengers for help during the flight.
Blimey, sorry for the essay!
Think night flights are easier, the noise seems to soothe them. Have fun!