Good girls guide to murder, is the story all finished in the first book or do I have to buy all three books in order to fully understand the story?

Please no spoilers about the ending and twists!
So I just bought the first book and I was wondering if this story going to be finished in one book and the two other books are about something else, or …? Also did you like the books? Which one was the best? Who is your fav character beside Pip? What did you like and dislike about the books?

by Voldi01