Just need a little help. I have acquired two party slots. How can the third party slot be available to me? Purchase, unlocking quests, etc.

Thanks in advance!

you have to purchase it, it will only be avalable after you unlock the Astril's vale tho

Slight correction, you can purchase the 3rd slot in the second region at the sanctuary, just like how you bought the second one at the haven. So you just need to play the storyline until you unlocked that area.