Fitting three car seats across one back seat
Three across is a width problem with a belt-access problem attached. What actually makes it possible.
Installing a car seat: LATCH or seat belt, and how tight is tight
Both methods are correct. Which one you use depends on the car, the seat, and the combined weight limit most people never read.
Rear-facing, forward-facing, booster: the stages and what moves you between them
Every stage is defined by the child outgrowing the previous one, not by a birthday. How to read the limits printed on the seat.