For start il tell you in steps:
Step 1 : Go to your Diablo 3 folder. Copy it, to a new folder make a new folder like (Diablo 3 game 2). (This is done not to use the same Diablo files on both bots as it can get conflictual)
Stept 2: Enter the folder you just copied Diablo , right click on the Diablo III (Not the launcher, the executable) and select Send to -> Desktop
Step 3: Go to desktop Right click the file select Properties and there at Target line at the end of the line add -launch with a space betwen the last letter and -launch
Example: "D:\Games\Diablo contru 3\Diablo III.exe" -launch
Hope it helps and i was clear.