Hi Molly:
By "booster" do you mean a forward facing seat with a 5 point harness that can later be used as a belt-positioning booster when the harness is outgrown by height or weight (these are called "Combination" seats)? 3 years is too young to be using a belt-positioning booster (seat that has no harness and just positions the seat belt on the child).
For your son's age, a good choice would be the Graco Nautilus, which is a forward-facing seat that can be used with the 5 point harness up to 65 lbs or about 18-19" of torso height (seated distance from the child's rear end to shoulder), then can be used as a belt-positioning booster.
WHat seat does your child currently use? The only convertible seat on the market that can be used *with the harness* to 80 lbs. is the Sunshine Kids Radian80 or XT and if your son is "almost too tall" for his current seat I'm guessing it's something like a Cosco/Eddie Bauer/Safety 1st Alpha Omega "3-in-1" type seat, which has a 40 lb. weight limit on the 5-point harness and then can be used as a belt-positioning booster. If you do have the 3-in-1 seat, please check that your son's shoulders are at or below the harness slot because otherwise he's outgrown it by height. Also for many of the 3-in-1 seats, the top slot is only for when the seat is used as a booster b/c it's not reinforced for use with the harness.
an awesome resource for all car seat questions is http://www.car-seat.org/ - many folks there are certified child passenger safety techs and all are really helpful and knowledgeable at helping you find a seat that fits your child, car, and budget.
edited to add: another option would be to get your son a convertible with a higher height/weight limit (such as an Evenflo Triumph Advance, which has a 50 lb. forward facing harnessed weight limit and a height limit similar to the britax marathon, and can be found online for about $110 with free shipping at places like www.albeebaby.com ). Then you could pass your 3YO's current seat down to the baby, and by the time the baby outgrows the old seat your 3YO should be old enough/mature enough for a belt-positioning booster and then you could pass the larger convertible down to your younger child). Feel free to email me directly if you have any more questions!