The Cosco Scenera 5pt is a good but basic seat and great for travel because it's fairly light weight. I would worry that it
may already be too small for your 3yr old though. My ds outgrew this seat by height at that age (tops of shoulders past the top harness slots). Another seat that you might consider for travel is the
Evenflo Chase, which is a fwd facing only 5pt seat that harnesses to 40lbs, top slots are a couple of inches taller than the Scenera's, and it also converts to a poor booster seat (due to closed belt guides). It is a less expensive seat though and fairly light and narrow too. If your ds is on the longer torso'd side like my ds was at that age, you might also look into that seat.
The Compass is a recommended booster on car seat forums. You will probably have to check it unless it folds up small enough to fit in the overhead bin (you might want to check the airline website for carry on measurement info).
The Cosco Pronto is less expensive but keep in mind it does not have a reinforced back and it requires the full support of the vehicle seat behind the child. It's been a while since I checked this out IRL, but I believe the wording on the sticker says it is outgrown by height once the midpoint of the childs head (ie, tips of ears) are past the top of the vehicle seat back. Chances are your rental car will have tall enough seat backs but this is the reason I would skip it because you just don't know when you'll have compatibility issues due to a lack of a vehicle headrest.
FWIW, I was always very leary of checking my car seats because you never know if a 50lbs bag (or several) will be placed or thrown on top of it, and what kind of unseen damage that could potentially do to the seat. We always used ours on board the plane and ds always seemed very comfortable in his car seat at that age.
Edit to add: here is a link to what the Florida State Hwy patrol recommends for car seat use in Florida:
http://www.flhsmv.gov/fhp/CPS/4Steps.htm
hth