Parallel Storylines

So I'm writing this Fantasy/Action story with 3 parallel story lines and a 3rd person perspective. Two of those story lines happen in the present and one in the past. I'm not sure If I should give each story line a whole chapter, or scatter them throughout the chapters.

(Any additional advice on parallel story lines with a 3rd person POV will help too)

What type of parallel story line are you talking about? Are you using multiple perspective (with each story line attached to one character), or is it a single narrator with three plots relating to themselves, or is it a single narrator with three plot lines relating to them and their group of friends? Each context will have a different answer.

Each storyline has a character. The two in the present are kind of woven together. The one in the past is related, but has a plot on its own.

So next question: do you have point of view changes to each character, or is it only one point of view?

Yes, there are changes in point of view between characters. But I'm not sure if I should wait a whole chapter to make that change between the characters or if I could do it mid-chapter.

(Looking back at my first question, I noticed I wrote it in a way that may have made you misunderstand it. This is the actual question I'm looking for an answer to)

Either way works!

For now, I'd switch whenever you feel there needs to be a switch. Figuring out the nuances of PoV switches is something that's best for later drafts as you polish up the pacing and really get the flow of everything right. For now, just write it however you see fit. The best practice for how to do it will probably come when you edit.

One thing I would advise against is doing in-scene switches. It's really tempting to just slip into another character's head when you're writing, especially close third, but it's a temptation I would avoid. It can get really confusing to readers when you're drifting from head to head without the transitioned handled with a really good hand.