I think I see the problem when I look more closely at your first screen shot--you've imported (mounted) all three of the drives. Detach them (in the Active Volumes screen, click on one of the drives, then on the icon at the bottom with the red X--repeat with the other two). Then you should be able to use the dd command to wipe them. This will, of course, destroy any data on the drives. What are you planning on doing with three relatively-small drives of different sizes?