Boosters are NOT meant for 3 years. 3 year olds do not have the bone development to take the force of a 3 point seat belt. Booster seats just sit on top of the vehicle seat and the 3 point seat belt (has to be a lap AND shoulder belt) is used to restrain the child. A booster's job is to boost the child up so that seat belts fit older children. An ill fitting seat belt is very dangerous and has killed many children. Booster seats are very safe, but for older children, NOT 3 year olds. Children should be in boosters until the child can past the 5 step test (in sources)
The MINIMUM guidelines to use a booster seat is when ALL of the following apply:
-the child is at LEAST 4 years old
-the child is at LEAST 40 lbs
-the child has outgrown their forward facing 5 point harness car seat*
-the child is able to sit still as required by a booster seat for every single ride- if they cannot sit still then you need to buy a car seat with the internal harness; this means they cannot play with the belts, lean over to tease a sibling or to pick up a toy. I don't think a 3 year old has the maturity level required to sit still. Many 5 and 6 year olds still don't have that maturity, which is why they should remain in a 5 point harness car seat.
All of the above requirements must be met until your child can safely transition to a booster seat.
*A forward facing car seat is outgrown when ONE of the following happens:
-the child has hit the maximum weight limit of the seat forward facing with the harness
-the tips of the child's ears are even with the headrest or shell of the seat
-the harness is beneath the child's shoulders*
*When forward facing, the harness straps need to be at or slighly ABOVE the shoulders to prevent compression of the shoulders. This is why a car seat is outgrown when the shoulders are above the top harness slot. This is why you should buy a car seat with at least a top harness slot of 17 inches.
Car seat manufacturers like to advertise their booster seats for 3 year olds. It may be illegal to use a booster seat for a 3 year old, depending on your state. In the state that I live, your child must be at least 4 years old to use a booster seat. But remember that laws do not represent best practice- don't think that the government will protect your child.
Most parents wait until their child is 5 or 6 years old to transition to a booster seat. This is because the pelvic bones had a longer chance to develop to take the force of a 3 point seat belt. If you were to get into a crash, your collar bone could break or crack from the pressure of the seat belt. Can you imagine what this force would do to a child? Children are not mini-adults. They have different bone density. The bone density doesn't resemble that of an adult until age 13. This is why all kids under age 13, no matter how tall or how heavy they are- these kids need to sit in the back seat. This is because they cannot take the force of an airbag. The airbag would crush a child's bones resulting in severe injury or death.
Some forward facing 5 point harness car seats that I would reccomend:
Evenflo Maestro (50 lbs forward facing; 18 inch top slot) About $90
Graco Nautilus (65 lbs forward facing; 18 inch top slot) About $140
Britax Frontier 85 (85 lbs forward facing; 20 inch top slot) About $299
I would get the Evenflo Maestro or the Graco Nautilus unless your daughter is super tall or heavy for her age. Sometimes you can find deals at amazon.com but be sure that you buy the seat brand new.
When you install a forward facing 5 point harness car seat:
-always use the top tether for the seat whether with the lower anchors or seat belt
-install it with less than 1 inch of movement- if that were my child that seat would be in rock solid
-chest clip at armpit level, not over belly
-harness is snug so that no belts can be pinched- if you can pinch the belts then they are too loose
Just remember that car accidents are the leading cause of injury, disability and death for kids under 12 years of age. http://www.youtube.com/watch?v=6RYgLnmMxv0
Above link: IIHS video about forward facing 5 point harness car seats; at about 1:23 the girl is using a Britax Frontier 85 http://www.youtube.com/watch?v=LhLDyAmZbPU&feature=relmfu
Above link: IIHS about booster seats