You have to be slightly careful with the WD Caviar Green drives, sometimes the firmware on them isn't compatible with the Time Capsules and so they don't spin down during idle time.
They may have solved this problem now, but it was a problem with the older smaller capacity drives. That's why I ended up with a Samsung EcoGreen. Would be interested in finding out how you get on with your 3TB WD drive!
http://forums.macrumors.com/showthread.php?t=1059249