It might seem they would, but there are some major considerations -- such as the fact that incompatability + misuse likely to be even more common.... Not to mention that a 3 year old's safety seat needs are different than a newborns, etc.
What vehicle(s) year, make, model?
What is your 3 year old's current seat?
What is your 3 year old's actual weight & height?
What is your budget?
I'll ask around about discounts for parents of multiples
(Babies-R-Us gives a 10% discount, but still finding out about more: have you asked around some twins forums?)