A full program printing the result.

Try it online!

How?

_2µ“€ðБẋ“¡Ø¤“©ßµ‘js3x€2¦€’+⁽"7ỌY - Main link: n
_2                                - subtract 2
  µ                               - start a new monadic chain with n-2 on the left
   “€ðБ                          - code page indexes [12, 24, 15] (middle row characters)
        ẋ                         - repeat n-2 times (make unexpanded middle rows)
         “¡Ø¤“©ßµ‘                - code page indexes [[0,18,3],[6,21,9]] (top & bottom)
                  j               - join (one list: top + middles + bottom)
                   s3             - split into threes (separate into the unexpanded rows)
                          ’       - decrement n-2 = n-3
                        ¦€        - sparsely apply to €ach:
                       2          -   at index 2
                     x€           -   repeat €ach (expand centre of every row to n-3 chars)
                            ⁽"7   - literal 9556
                           +      - addition (0->9556; 12->9568; etc...)
                               Ọ  - cast to characters (╠; ╔; etc...)
                                Y - join with newlines
                                  - implicit print