Yes, we're back from our trip and my SIL did end up buying her daughter a seat. After they bought their tickets at $155 each roundtrip, the fare went down to $80 each roundtrip, so they rebooked and had a credit so they used it to buy my niece a seat. Really, at $80, I could have afforded to get it for her myself if they hadn't, so she was getting a seat either way. They checked her carseat since they saw no need to bring it on (which I guess at ~36 pounds is fine), but then never used to to/from the airport anyway. They said she sat in the plane seat just fine so I'm guessing that your 3yo will be fine in the seat, NHAnn.
Also, they flew up to NJ 2 weeks later for our wedding and booked those flights through a
travel agent, who made them get a seat for my niece. Her then 9 month old brother flew as a lap baby though (he didn't come to Disney World with us so he wasn't an issue). Since it was my SILs family, my MIL, FIL, BIL, and other SIL, they said there were plenty of them to take turns holding him so no need for a seat. They checked both car seats then and did use them in the cars this time. It's amazing that they buy Britax carseats for their safety, but then skimp on safety in so many other ways. Oh well, there's no point in forcing my views on others.
We got my then 3 month old daughter a seat, and got her one on the 3 flights she's taken since then, but everyone's entitled to do what they want. Oh yeah, after having her seat on their once, we will always have it, it was so nice having a place to put her rather than holding her the whole flight, so i could use my tray for my pretzels and water. Also, on every flight she's taken we've gotten a half price ticket for her (on US Airways and Continental). (Except for Independence Air since the ticket was only $91 rt anyway.) Another bonus was that I was allowed bags and carryons for my daughter since she paid for a seat. This was great so that I could bring my electric breastpump to my mom's house, plus her diaper bag, and a large bag of photos. Not that I wanted to carry all that, plus Madison and her carseat, but I had to and it was nice to be able to.