Hi there. Sounds like a fun trip!
What seats will your kids be riding in on the plane? At 33 pounds and 20 pounds it's unlikely the plane's belt will fit any of them, and if it doesn't they'll be unprotected in case of turbulence or a runway emergency. The FAA recommends using a seat on board from birth to 40 pounds.
http://www.faa.gov/passengers/fly_children/
I'd probably get three Safety 1st Guide 65s. They're fairly narrow, inexpensive, and lightweight. All three seats would cost $240 and weigh 36 pounds.
http://www.walmart.com/ip/26845783?...87616836&wl4=&wl5=pla&wl6=55361902236&veh=sem
As for a car, a BMW 5 series would probably work for all three, or a Ford Mondeo. A Skoda Octavia can probably fit three Guides as well (it fits a size 8 adult, a First Years True Fit rear facing, and a Clek Ozzi, though it was a tight squeeze on my hips). I'd probably avoids Fiats, I don't think they'd be wide enough.
Practice installing your seats with a locking clip before you go, that way you can install them in the European car. Their cars don't have locking seatbelts, and you can't use LATCH for all three. And in a tight squeeze, three seatbelts are usually easier.
As an American tech, I had a tough time understanding European instruction manuals. They're nowhere near as extensive as ours, usually some pictures and a few words in about eight languages, and they don't install the same way ours do. They also generally top out rear facing at 9kg, unless you got some 13kg 0+ seats (ordering online you would also have the option of Swedish 25kg rear facing seats, but those are pricey and I don't know if you could do three across. Maybe). Most harnesses top out at 18kg. So you'd have two forward facing and one rear, and that one rear would be hopefully rear facing, and likely in an infant seat of sorts. And like I said, I found them all confusing to install. We lived in Germany for six months last year. Most seats are constant throughout Europe, but there is a possibility that the Italian seats would be different from the German ones.
I'd really stress that you take three American seats with you on board the plane for the safety of your kids, and the ease of installing seats you know how to install and have practiced with before you go.
HTH
Wendy