the car hire company will have a car seat for your youngest, I'm not sure about booster seats, but they will charge you $5 per day, per seat, and the condition of the seats is terrible,
We always take our own - you can check them, and collect them with your luggage - one car seat has never been a problem, but 3 maybe more tricky, but if you can cope, then I would suggest taking them.
the other option is to buy some from Walmart, but that involves driving to Walmart without a seat, which is obviously not ideal.
Last time, we took a seat which splits into a booster and a back - we checked the back, and took the booster onboard - Katie sat on it, and found it made it easier to use the tray etc. We only had the one child, and were able to hang the booster on the back of the pushchair so it was not a hassle, but I would imagine with 3 kids, and 2 boosters, then that is probably not so easy.
Bev