Within the text of a novel, it’s best to use contractions. Here are a few:
Its or It’s
The contraction of it is = it’s
The possessive term of it = its
It’s my Birthday.
My dog wags its tail.
I leave my house by its front door.
There is no such term its’
Who’s or Whose
The contraction of Who is = Who’s
The possessive word of who = whose
Who’s paying the bill?
Whose house is this?
They’re or Their
The contraction of they are = they’re
The possessive term of they = their
Bill said they’re his clothes.
The problem is their fault.