Hey, new to the game, need a bit of help.
I got my first new slot, but how do I get slot three?
I on dealing with a powerful boss with three opponents and figured one of my other girls would be a good help. You know... make things more fair and all that. But the area where its at is in grey... so I can't pay for the new slot.
I don't get it. Is another event needed for unlocking the third, or am I just missing something?

You can open the 3rd slot by going to the sylesti plaza and paying 25k to open it.
However, I think you have to be past a certain part of the main quest to be able to open it. x: If the option is grayed out for you, that means you'll have to beat the boss using only 2 pets until then; after that, you can unlock it.

Hm, that sucks. Thats a lot of money... and that means I'm really gonna have to train.
Well, thank you for letting me know~. Much appreciated.

Just so you know, you can add a third party slot when you get to Umbra Forest. But for a cost of 25,000.

Missions are a great way to earn gold and experience for your pets. You can add an extra mission to your limit at the plaza as well, trust me when I say it's worth it. You can also find gold exploring, and Jinxie's Jamboree is another slow but generally sure way to earn some.