Failure of any item of Section 3 means a failure of all Section 3. So long as you don't fail any other section in the Skill Test you will have to retake the whole of Section 3 that will also include the planning, departure and arrival phases.
You cannot fail 3b and simply repeat 3b with the rest of the test! If you do it will not be accepted.
Standards Doc 20 tells the examiner what to do. Para 7.4.1 states: A FAIL in Section 3 shall also require a repeat of Section 1 and items a, b, and h of Section 4.