You won’t be able to manipulate the courses HTML wrapper since an exit button is going to appear in web-based content no matter what.
You could manipulate the internal course exit button with a variable.
Create a User Defined Variable
Activity one completed = modify the variable to add a
Activity two complete = modify the variable to add b
Activity three complete = modify the variable to add c
Title level action that if variable equals abc show exit (the key is to make sure that no matter where the person is they have a chance to refresh the variable value. Title level would only do it once the page is shown. So I would suggest adding this action to the page where the activities are to force it to retrieve the variable data after the completion action has been done)