In you are looking for an equation that goes along with this statement, it would be written:
3b + 6
b represents the number of balls in each bag. Since we do not know the number, we just put b down. However, we do know that no matter how many come in a bag, we have 3 times that many since there are 3 bags. So multiply the 3 and the b. In the end, you add 6 since there are six more than the 3b amount that we started with.