1 Hp : 1,150
The Hp of the third Black Waltz is 1,150. Mot much more than that of Black Waltz 2 but he is more difficult to beat.
2 Lots of potions needed
The fact that you will have to fight him without Garnet, as she will be driving the ship, will make things harder. Make sure you have lots of potions.
3 Stealing is hard
Stealing from that Black Waltz will be hard since every move you use will count. If you still insist to steal then those are the items you can get: Silver Gloves, Linen Cuirass, Steepled Hat.
4 Phoenix down needed
Since some of the attacks of that Black Waltz can cause a lot of damage you might need to use some phoenix downs to revive dead members of the party.
5 Use Vivi's double magic
As Vivi will be in Trance state his magic will be performed 2 times each turn. This is a good way to attack Black Waltz 3 since he won't counter attack.
6 Use Steiner's magic sword
Use Steiner's magic sword to cause damage to that Black Waltz.
7 Let Zidane heal the party
Most probably at this point Zidane's attacks will be the weakest in the party. Use him to heal the party members using potions.
8 The bandit ability can help with stealing
If you have the Bandit ability then stealing from Black Waltz 3 might be a lot easier. However, it would still be very hard to steal everything he has. See also how to defeat Gizamaluke.