The third user will not be allowed access, because both of the two assigned CALs are allocated to the two users, that had opened the document earlier. You'll need to manually remove at least one of the assigned CALs, before the third user can access.
Dynamic CAL assignment lets you assign CALs du the users without explicitly naming them. Whenever a user request to open the document he/she will get a license assigned - that's if any is available.
"So, my question is if today 2 users have opened the document and after 24 hours if 3rd user opens the document, will the Doc CAL from 1st two users will automatically be freed and assigned to the 3rd user? Or do we need to do it manually?"
They´re not release automatically, but you can use Power Tools 1.2 for QlikView to remove them