Time-outs may vary due to the severity of the offense, but should generally be at least one minute per year of age. Thus, a three-year old would be made to sit quietly for three minutes, at the least, for a violent offense. You, yourself, will want to model this behavior, by making a fuss over the child who has been injured. Be sure that all the positive attention you can muster is given to the offended child, not to the offender. They all have inappropriate behaviors, but they need you to teach them how to behave appropriately.
Violence teaches violence.
Be sure that you are calm before you discipline the aggressor. Use time-outs, positive role modeling, and discussion to change the behavior. Limit the amount of time your preschooler watches television, plays video games, or uses the computer. Many times children encounter fantasy versions of violence in these mediums, which desensitize them to real violence. For example, a cartoon character hits, kicks, or punches another cartoon character.