Buy an external caddy to hold the disk drive. Make sure it can take your size drive with the right connector (probably SATA)
Then on the Windows 7 PC download and install CCleaner. Within CCleaner is a "Drive Wiper" option.
Plug one of the hard drives (in the caddy) in to the Windows 7 PC.
Start up CCleaner, select Drive Wiper (under Tools).
Select the external drive from the list of drives displayed.
Select "Entire drive" (as opposed to Free Space Only).
Leave the "pass" as "1 pass". Note you can select 3 pass, 7 pass, even 35 pass, but the more you do the longer it takes.
Start the wipe and it will overwrite all the data on the disk in the caddy.
Note on a 3Tb drive this could takes AGES, possible all night, so kick it off just before you go to bed.