You have come to a point in Chapter 2 of the game that allows you to use other main characters classes, like when Noah was able to use the class of Mio, Eunie with Taion and Lanz with Sena. These swapping of classes also activates the Master Arts wherein you can combine art of your own with the other swap’ class allowing you to have a new new special attacks against the enemies. It’s like combining different Pokemons but for art.
What is Fusion Arts
Basically, the Fusion Arts is a art fusion where you can fused your character’s own art with other class art from your party. In the initial tutorial for the Fushion Arts, you will only allowed to use the main character partner’s class, but upon reaching rank 10 of the latter class – you’ll be able to use other class like using Medic Gunner, Tactician, Heavy Gunner, Ogre and Wide Slash.
It is best recommended to max out all of the classes available to your character to maximize the experience points you are getting while levelling up your character. So, make sure to check out the rank if its reaches the maximum ranks. During the initial, the max rank is 10 and the next max rank will be unlocked on the further chapter.
How to use Fusion Arts
First, you have to select the fusion arts for your character.
To use Fusion Arts in the battle, you need to recharge the art of both of the Master class art (Left corner – diamond shaped art) and swapped class art (Right corner – circle shaped art) that are in the same position.
Once it is recharged, you need to hold the 
ZR button and press the Swapped Class art button (From the right corner) to activate it.
When you reach character level 20 and level 40, the other slots for master art will be unlocked and you can use it to deal fusion art.
That’s it! Hope this simple guide helps you understand what is the art fusion and how to use it in Xenoblade Chronicles 3 (XC3)