Darik's Boot and Nuke (DBAN) indeed will do it but it is going to take a long long time to do 3 drives. That's a lot of electricity running a computer that long. One other wrinkle is that if the drive(s) have any bad sectors, DBAN will barf on that because it is trying to write zeros. It has been years since I tired DBAN. Now I just remove the screws to expose the platters and give 'em smack with a hammer (protect your eyes, some platters are glass). If you want to use the drives again personally then you can just erase and write new data on them.