jmc

Let $n$ be the number of Dummies in one bag. Then we know $n\equiv 7(\mathrm{mod}9)$, so $$3n\equiv 3(7)=21\equiv 3(\mathrm{mod}9).$$Thus, when the Dummies in three bags are divided equally among $9$ kids, there is a remainder of $\boxed{3}$ leftover pieces.

We can also explain this solution without using modular arithmetic. Each bag can be divided equally among the $9$ kids with $7$ pieces from each bag left over. This makes $21$ leftover pieces, which are enough to give each kid $2$ more candies and have $3$ candies left over. Those last $3$ candies can't be divided equally among the kids, so the answer is $\boxed{3}$.