A 3y can sit in a highback booster if they meet the weight/height minimums. My 3y neice meets these, but my 4y daughter is just meeting them. So read all of the minimums carefully.
There are some really good highback boosters in the $100 - 150 range that will take you to 100lbs so it should be the last one you'll need to buy.
Also, if your sitter doesn't drive your daughter around often, just as a as needed, I would probably find something on sale. My mom was a backup sitter for my daughter and when we bought a convertible seat for car I found a new one on sale for $30 at Toys R Us. She ended up using in only about 5 times in 4 years so I'm glad I didn't spend a ton of money on it.
My car has a Britax Marathon, because I am the one she is with most often, and I wanted a girly purple pattern. Literally, that was the only one at the time that had a pattern that was girly!
The convertible seat in my hubby's truck is black and we bought if for $100 at Target. He didn't want a girly seat in his truck!
Another trick is to look at the season change. Each season the companies come out with new styles and patterns. The old season patterns usually go on a really good sale. Which is how I found the seat for my mom's car.
M.