How do I combine multiple stories into one story?

Hey all. I've been working on a series that's now at three stories, with a fourth on a way. Recently, I've been considering just combining them all into one overall story. After getting a review from someone suggesting the very same thing, I think I should go for it. However, I'm not 100% sure how to and have some questions.

- I'm trying to figure out how I'm going to break the one overall story into multiple stories. Are they considered different 'Parts' at that point? - I believe I should be able to give each part the name of the story it was taken from as well (and not just be like 'Part II')? - I'm not sure if the chapter number is exclusive to each part or if the chapter numbers are in sequence with the entire story.

Any help is appreciated. Thank you.