Take this release http://musicbrainz.org/release/ac9d2158-b66c-4209-a2a2-e26b54e7c393/edit
Using advanced editor and following http://www.discogs.com/release/2034004 as a source, add disc 3 from existing MB tracklist.
This disc is added as disc 28, move it by hand (yes, enjoy!) to disc 3, doing that you'll have changed all existing discs position, so go fix them.
Done?
Ok, finish, go to the edit note tab, and you'll learn that:

• you've moved new medium. Ah? I thought I just added it... Furthermore it has never been at position 6...
• all existing discs have their position changed from 15 to 15, 16 to 16, ...

Verbatim copy:

Reorder mediums
Mediums: Disc 3: (moved from position 6)
Disc 15: Instrumentaux, Volume 1 (moved from position 15)
Disc 16: Instrumentaux, Volume 2 (moved from position 16)
Disc 25: Documents duos raretés, Volume 1 (moved from position 25)
Disc 26: Documents duos raretés, Volume 2 (moved from position 26)
Disc 27: Documents duos raretés, Volume 3 (moved from position 27)