You can fit three Graco SlimFits in there, as a Corolla owner I've done it. One has to be rear-facing.

It wouldn't be my primary car of choice for triplets, but plenty of families of five don't have three in the bulky kind of car seats simultaneously.

https://www.deserttoyota.com/toyota-camry-vs-toyota-corolla/ says the Camry is 72.4 inches wide, the Corolla 70.1 inches.