Cars and Fitting 3 Carseats...

Go for something with 3 rows. Enclave, explorer, Durango (might not be standard?), journey. I know there's more, but ??

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Renting a large luxury sedan should work. Toyota Avalon, Chrylser 300, Chevy Impala, Kia Amanti....I have three sunshine radians 5 pt harness forward facing and one rear facing in the back of my honda accord, tight but it works safely.
I just read your previous ? so I am adding more here. The sunshine radians are the slimmest seats on the market with excellent ratings with Highway Safesty the ones we have are 5 pt harness up to 65 lbs and 57".

I think it depends on what type of high back booster you are looking to use. I think the ones that use a regular seat belt are smaller than the 5 point harness boosters. My sister didn't use the high back boosters, but we were able to get two seat only boosters and a Britax Boulevard, which is a big carseat across the back of my Honda CRV and eventhough it's a SUV, it's a small SUV and I think the back seat is the size of a regular sedan.

i had to get a big suv ..nissan armada..and it's snug

I think just about any sedan will fit them. We have fit 3 carseats in a Dodge Intrepid, Toyota Corolla, and several others. As long as it's not a compact, you are probably fine.

Well, it's tight but I get two high back boosters and booster with no back in my Nissan Altima but it's very tight. I have a hard time hooking the seat belts but it does work. I would imagine there are other cars a bit bigger that would manage okay for a rental.

Maybe a Chrysler 300?

A ford edge with third row.