Which 3 car seats fit next to each other?
3 narrow booster seats
Are all your 3 children older than 4 years? Choose 3 slim booster seats with a backrest, so they can safely fit on the backseat next to each other. Most cars don't have an Isofix attachment for the middle seat. This way, you can only attach this car seat with the 3-point belt. This is no problem with a booster seat. The Maxi-Cosi Morion is an example of a slim booster seat. It's known that 3 of these seats fit together on the backseat.
Booster seat in the middle
Do you have 2 children who still use a toddler or baby car seat and 1 child of 6 years or older? The eldest child fits on a booster seat without a backrest between the 2 other car seats. A downside of this setup is that it's hard to click the seat belt of the middle seat into place.
Baby car seat on front seat
When all 3 your children sit in a baby or toddler car seat, they won't fit next to each other. Most cars don't have an Isofix attachment in the middle. In addition, the manual often indicates you can't place car seat in that spot. In this case, you can place 1 of the car seats on the passenger seat and the passenger can sit in the back. It's important that you disable the airbag of the passenger seat.