Don't give in! You'll regret the change. It may seem hard now, but it gets better... Normally by the time a 3rd one arrives, the oldest is almost ready for just a booster. Besides, the youngest should be rear-facing for at least a year - It's easier to squeeze a rear-facing seat between 2 forward-facing ones. Child seats definitely vary in size. The biggest tend to be the "1-seat-does-it-all" type, in my experience. If you avoid those you can still get good "mileage" out of specific weight category seat as you'll have
3 kids going through it.btw, the Europeans are probably laughing right now having raised 3 kids in the back of a Lupo!