The first problem is having a booster for a 3 year old. CDC says a child must be rear facing until at least 2, in a forward facing car seat from 2 to AT LEAST 5, a booster from 5 until they are 57 inches tall. A child under 5 does not have a well enough developed spine. When in a booster too early and in an accident the spine will compress typically causing severe spinal injuries.
Auto related injuries are THE leading cause of child deaths. The largest % being associated to improper seating.
Read here before doing anything and protect your childen. I personally see 90% of people without proper seating and/or rushing their child through seating to convenience themselves selfishly. If my children were to get injured by my poor choices of child seating I wouldn't be able to live with myself.
http://www.cdc.gov/motorvehiclesafet...factsheet.html