Each one is about something different, but the first two focus on a certain character as the main character named Yuri. The first one you have to kill these three gods with your teammates and Yuri's fusions (he can fuse with monster and become powerful fusion monsters) and save the world (go figure
).
The second is still focused on Yuri, but he loses all of his powers to the misletoe curse and is slowly being killed by it. You have to go and get the help of the god of wrath, Amon, to resist the curse and save the world from the people who cursed him and plan to take over the world.
The third.....I dunno. I'll let Gondor take that one.