code snippet

List<GameObject> targets;
int targetMax=3;

OnTriggerEnter(collider c){
    if(targets.Count<targetMax && !targets.Contains(c.gameObject))
    targets.add(c.gameObject);
}

OnTriggerExit(collider c){
    if(targets.Contains(c.gameObject))
        targets.remove(c.gameObject);
}

Update(){
    //fire on enemies
    foreach(GameObject go in targets){
        Shoot(go);
    }
}