You can run them all from the same folder, although people have varying experiences with speed. If possible run your accounts from different hard drives.

No need to reinstall WOW for the other 2 copies... just copy the wow folder and rename it:

C:\WoW
C:\Wow2
C:\WoW3

or like mine:

C:\WoW
P:\Wow2
Q:\Wow3
R:\Wow4
S:\WoW5