Of course it is. Either buy something like ISboxer, or copy your entire Diablo folder to another location in your drive.
For example:
1st Diablo: C:\program files\diablo1
2nd Diablo: C:\program files\diablo2
3rd Diablo: C:\program files\diablo3
Make a shortcut in EACH of those folders, and copy them to desktop. right click on them and add "-launch" at the end of the target line. Now click on each in turn, and voila: three copies of Diablo 3 opened.