I'm so glad you are looking out for your son's safety!
There is SO much misinformation out there...most of it well meaning. Please keep in mind that the internet is not always the most accurate source for information. I was a certified child passenger safety technician for 4 years--just let my certification expire in October because I didn't have time to do the recert-requirements.
Anyway...I wrote a response to the "David Miller-5 point harness" u-tube video on my blog at http://knittedinthewomb.blogspot.com/2006/12/5-point-harn... Bottom line...the intent of the u-tube video is to be helpful, but it isn't entirely accurate.
I see MANY people in this thread who seem to be using seats (like the Eddie-Bauer 3-in-1) with a harness that has a 40 lb limit well beyond 40 lbs. The first and most important rule of carseat use is READ THE DIRECTIONS AND FOLLOW THEM. So important.
I've also seen statement that high backed boosters are safer than low backed. This simply isn't true either. In some situations, low backed boosters are safer, because they position the child further back on the seat, thus reducing the chance of the child hitting the seat in front of him. But if the vehicle seats are not high enough (at least to the top of your child's ears) then you need a high backed booster to protect against whiplash. Also, with younger children who fall asleep you may need the high backed booster to keep the child properly positioned in the vehicle seat belt.
A 3 year old is typically too young for a belt positioning booster--they will just move around too much and not stay well protected. In this case, as some posters have said, there are seats available (for a hefty price) that have a harness designed for a higher weight limit. These are an excellent choice. If they are out of your budget, then you can try "locking" your seat belts. You do this by buckling your child, then pulling the shoulder belt the whole way out. This will turn the belt (in some vehicles--not all) into "locking retractor" mode, or "boa constrictor." It stays tight on the child, so the child can't move as much.
Hugs & good luck!