3-year-olds (and young kids, in general) lack the maturity to stay properly seated for every ride in a booster seat. Even though some booster seats are rated down to 30 lbs., it is generally accepted that children under 40 lbs. are best protected in a seat with a 5-pt harness.
In this way, what are the ages and stages of car seats?
All children whose weight or height exceeds the forward-facing limit for their car seat should use a belt-positioning booster seat until the vehicle seat belt fits properly, typically when they have reached 4 feet 9 inches in height and are 8 through 12 years of age.
When should a child be in a booster seat?
You can safely switch your child to a booster seat if she's at least 4 years old and she weighs 40 pounds or more or has grown too tall for her car seat (when her shoulders are higher than the top set of harness-strap slots in the car seat's back).