Help! So I have a 21 month old about 24 pounds and about 32 inches tall in a graco 4ever dlx rear facing. A 3 year old that is roughly 37ish inches and 37 pounds ( on the small side) in a cosco finale 2-1 harnessed. And a 5 year old that is 43 pounds and about 43ish inches tall also in a cosco finale 2-1. However, the 5 year old is getting close to maxing out her current seat. And I need 3 across in a 2007 Toyota Camry. The 5 year old is booster ready and sits maturely during the entire ride. I am open to a convertible seat for the 3 year old to put him back to rear facing.
Hi Kim. Honestly, with a 3-across, it's better to leave everyone harnessed. It's super hard trying to fit a hand in between seats to get a latchplate buckled for a booster rider, plus there aren't any narrow boosters that are less than $199. Of course, the solution I'm about to give you is more than $199, so . . .

.
The 4Ever DLX is a pretty wide seat to try to do a 3-across and I'm guessing that's why you need the Finales. Are they all fitting properly? That means when you install them separately, they move less than 1" at the belt path. You don't want any of the carseats depending on the others to keep them tight.
If they don't fit properly, the 4Ever probably needs to go. Graco makes a really narrow carseat, the SlimFit3 LX (
https://amzn.to/3FRdlDv) which is also called the True3Fit LX at Walmart (
https://fave.co/3gM2m2k) that is a great all-in-one. Your 5-yr old could probably use it harnessed for quite some time.
If you like the 4Ever and only want to buy a new booster for the 5-yr old, I'd look at the Maxi-Cosi RodiFix (
https://amzn.to/3pLiMy8) or the Peg Perego Flex 120 (
https://amzn.to/3sQW7m1). Neither of these boosters have arms so it's easier to buckle them and they're both quite tall so they'll last for a long time. They are highback only.
HTH!