And car seats! Three kids are dramatically more expensive than two because you need a bigger car, and many family activities are priced for a family of four.
Perhaps not as big a deal in the US, but in the EU where some form of car seat is required for the first 8-10 years of a child's life it's a big thing.
That only applies to 3 at the same time. And most Americans drive SUVs and other large cars regardless of how many children they have. Small sedans basically aren't even sold in America anymore due to lack of consumer preference.
Perhaps not as big a deal in the US, but in the EU where some form of car seat is required for the first 8-10 years of a child's life it's a big thing.