For each instance of Diablo you plan to run, create a new copy paste folder of your original Diablo folder.
Name them as you wish, #1 #2 #3 etc
Now go to each folder and locate the file named Diablo III.exe
Create a desktop shortcut for each folder and above named file.
Then go to each shortcut and add -launch to then end of each file, right click properties, Target
Example as follows:
"C:\Program Files (x86)\Diablo III 1st Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 2nd Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 3rd Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 4th Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 5th Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 6th Account\Diablo III.exe" -launch
"C:\Program Files (x86)\Diablo III 7th Account\Diablo III.exe" -launch
Provided you have a Diablo license for each, launch each and log in.
If using DemonBuddy, obtain the number of licenses to match you planned active accounts.
Final step:
Have fun!