Car crashes are the leading cause of both disability and death for children in the United States- and they are for a reason.
Just a few weeks ago, a 3 year old was killed while riding in a booster seat: http://www.ktnv.com/news/local/187038601.html
My greatest sympathies are for the parents- it really just takes an instant to loose a child.
The American Academy of Pediatrics (AAP) recommends that children ride in a forward facing car seat with a harness for as long as possible. The AAP recommendations say that moving a child to the next step is a decrease in your child's level of protection. In fact, the National Highway Traffic Safety Administration (NHTSA) says to keep 4 to 7 year olds in a car seat with a harness until they have outgrown it: http://www.safercar.gov/parents/RightSeat.htm
There are many car seats with tall top harness strap slots and high forward facing weight limits. Here is my recommended list:
-Baby Trend Fastback (18" top slots; 70 lb weight limit with harness)
-Evenflo SecureKids 300 or 400 model (18" top slots; 65 lb weight limit with harness)
-Graco Nautilus (18" top slots; 65 lb weight limit with harness)
-Graco Argos 70 (18" top slots; 70 lb weight limit with harness)
-Recaro Prosport (19" top slots; 90 lb weight limit with harness)
-Britax Frontier 85 (20" top slots; 85 lb weight limit with harness)
-Britax Frontier 85 SICT (20" top slots; 85 lb weight limit with harness)
We all know that car seats pass standards. But what are these standards? Federal standards say that a child's head can move as far as 32 inches from the vehicle seat back when installed without the top tether. The top tether is extremely important as it reduces this forward movement as much as 4-6 inches. If you were to measure 32 inches in your vehicle- you may be very surprised.
Every car seat on the market comes with the top tether, however, vehicles made after September of 2002 have factory installed top tether anchors. These anchors can be found on the rear shelf in a sedan, on the back of a captain's chair, or on the ceiling in an SUV.
The Britax Frontier 85 only goes up to 65 lbs without the top tether, however using the top tether is always recommended. The SICT model of the Frontier 85 does not require the top tether at any weight.
A booster seat just sits on the vehicle seat. It is not attached to the vehicle seat in anyway. Then you thread the lap and shoulder seat belt through the belt guides as said in the manual. A booster seat is designed to lift a child up for a better seat belt fit- by placing the lap belt on the child's hips and the shoulder belt between the child's neck and collar bone.
Not all booster seats are doing their job- some boosters put the lap belt on the child's tummy. During a crash, it will sink in, injuring the child's inner organs and spinal cord. The Insurance Institute for Highway Safety (IIHS) rates how well booster seats position the adult seat belt on the child's body.
Here are their results: http://www.iihs.org/research/topics/boosters/default.html
If you are going to put your 3 year old in a booster anyways, I would recommend the Graco Turbobooster in highback as it does a great job positioning the adult seat belt.
As a side note, all car seat manufacuturers prohibit the use of aftermarket products. This includes a child's bulky winter coat. This is because during a crash, the coat will compress, leaving the harness really loose on a child. This will mean that head excursion will increase, or in a severe case, the child will be ejected from their seat.