There are some expensive car seats designed to cram all three kids into a back seat, but buying a larger car instead is cheaper. They also tend to be designed for triplets, if all three kids are different sizes it becomes a lot more complicated.
Most cars only have attachments for two car seats in the rear, so even if you did have space the middle seat would have to use the generally discouraged seat belt install instead.
Most cars only have attachments for two car seats in the rear, so even if you did have space the middle seat would have to use the generally discouraged seat belt install instead.