Fives tripple counter?

If you attack him with Dooku (or any other multi-attack toon), he can counterattack each of your attacks. And if he lands speed down on his first counterattack, then his second counterattack will also get the bonus attack. So, if his turn is right after that, it is wholly plausible to see him get 5 attacks in a row (or more, if he is attacked by Leia 3 times) if he targets the same toon who has/gets speed down. That's a rarity, but it can happen.

As far as Fives being overpowered (as other posters have hinted), he is way down the list of characters who could be seen in that boat.