Try the following:
sudo diskutil mount /dev/disk0s3
df | grep disk0s3
It actually mounted the Recovery partition (on High Sierra), where you could create a .dmg from the mounted Recovery file system.
On the High Sierra system, I have access to, /dev/disk1s3 was the recovery partition (It is an SSD based Mac). When I mounted it, it appeared as /Volumes/Recovery and an ls on the mounted file system shows:
ls -Rl /Volumes/Recovery
total 0
drwxr-xr-x 3 root wheel 96 Dec 31 2016 ./
drwxr-xr-x+ 4 root wheel 128 Jul 1 17:34 ../
drwxr-xr-x 13 root wheel 416 Jun 23 15:15 80C07111-95EA-4191-830B-614E539AAE52/
/Volumes/Recovery/80C07111-95EA-4191-830B-614E539AAE52:
total 1063560
drwxr-xr-x 13 root wheel 416 Jun 23 15:15 ./
drwxr-xr-x 3 root wheel 96 Dec 31 2016 ../
drwxr-xr-x 5 root wheel 160 Jun 23 15:15 .diagnostics/
-rw-r--r--@ 1 root wheel 1984 May 29 18:53 BaseSystem.chunklist
-rw-r--r--@ 1 root wheel 485329910 May 29 18:15 BaseSystem.dmg
-rw-r--r--@ 1 root wheel 5082 May 17 23:41 PlatformSupport.plist
-r--r--r--@ 1 root wheel 480 May 29 17:03 SystemVersion.plist
-rw-r--r--@ 1 root wheel 576056 Jun 23 15:15 boot.efi
-rw-r--r--@ 1 root wheel 2738 May 29 18:12 boot.efi.j137ap.im4m
-rw-r--r-- 1 root wheel 397 Jun 23 15:15 com.apple.Boot.plist
-rw-r--r--@ 2 root wheel 29299999 May 8 19:01 immutablekernel
-rw-r--r--@ 1 root wheel 2738 May 29 18:12 immutablekernel.j137ap.im4m
-rw-r--r--@ 2 root wheel 29299999 May 8 19:01 prelinkedkernel
/Volumes/Recovery/80C07111-95EA-4191-830B-614E539AAE52/.diagnostics:
total 5568
drwxr-xr-x 5 root wheel 160 Jun 23 15:15 ./
drwxr-xr-x 13 root wheel 416 Jun 23 15:15 ../
-rw-r--r--@ 1 root wheel 328 May 29 18:53 AppleDiagnostics.chunklist
-rw-r--r--@ 1 root wheel 2839919 May 29 18:13 AppleDiagnostics.dmg
-rw-r--r--@ 1 root wheel 977 Apr 30 12:25 Info.plist
And then I was able to create a .dmg using
sudo hdiutil create -srcfolder /Volumes/Recovery Downloads/tmp.recovery.dmg
...................................................................................................
created: /Users/raharris/Downloads/tmp.recovery.dmg
ls -lh Downloads/tmp.recovery.dmg
-rw-r--r--@ 1 root staff 492M Jul 11 22:15 Downloads/tmp.recovery.dmg
Does that help?