I wish legal minimums were not seen as the same thing as best practice.
I'm so glad my tech class taught exactly WHAT state minimums were, but exactly WHY doing more than the minimum is safest.
That said, I'd never put a 3yo in a booster--not even a large 3yo. My daughter was 40 pounds and 42+ inches at 3, we did everything we could to find her a higher weight harnessed option, despite the fact that all the stores in this area only carry Graco infant seats, Cosco/Safety 1st convertibles (which are all WAY too short to get taller/longer-torsoed kids to a full 40 pounds without getting too tall for the harness first) and Graco TurboBoosters. So, since my daughter outgrew her 3-in-1 at 2.5 years old by height, I should have just moved her to a TurboBooster, or to the 3-in-1 as a booster. After all, she was 38 inches tall and 35 pounds. Thankfully I found this board and learned that if I travelled 2 hours I could purchase a seat that could still keep her harnessed...and she's 6 years old now, still fitting with years to grow in that seat.
Anyway, my long-winded opinion is that even IF it is legal, I wouldn't do it.
Have you seen crash test video footage of a harnessed seat and a booster seat?
http://www.oeamtc.at/netautor/html_...videos/test2002/frontcrash/maxicosipriori.mpg
http://www.oeamtc.at/netautor/html_seiten/kisitest_2002/videos/test2002/frontcrash/maxicosirodi.mpg
Those are European tests (since the US doesn't release test results) of the Maxi Cosi Priori (a forward-facing only seat, but it's similar to some of our US seats) and the Maxi Cosi Rodi (similar to our Turbo Booster, and I believe the Rodi is now available in the US but I haven't heard any reviews to know if it is exactly the same or not).
It's clear that, while a booster does provide more protection for a child than a seatbelt alone, a harness provides even MORE protection by providing two more points of restraint, in my opinion highly important especially for such a small child.
As a CPST, I can understand the need to put some kids in boosters at age 3, at community seat checks, for example. But it's a last resort, IMHO. Sometimes finances mean a higher weight harness isn't an option. Thankfully with seats like the Fisher Price Safe Voyage Deluxe, Evenflo Triumph Advance and Cosco/Safety 1st Apex/Biltmore, options are becoming more viable for lower income families.
