I looked for this today at Astro but couldn't find it.
If it's the same as the R1 40th Anniversary edition, then it has all-new documentaries - Julie Andrews & Christopher Plummer: A Reminiscence, From Liesl to Gretl: A 40th Anniversary Reunion, When You Know the Notes to Sing: A Sing-Along Phenomenon, Mia Farrow screen test, A Few of My Favorite Things: A retrospective documentary, a new commentary track with cast, choreographer and an actual Von Trapp, and more.
I think the documentary special features from the old set (Salzberg: Sight and Sound, The Sound of Music: From Fact to Phenomenon) all didn't make it to this new release.
Hopefully someone who's seen this in stores can confirm if the R3 is the same as the R1.