How can I schedule ClamWin to scan my 3 hard disks? Do I have to schedule it for each disk seperately? If yes what shoul be the time interval? Thanks!

Currently there is no option to "scan all hard disks".

The time interval, i guess, can only be determined by actually executing the scans.
I suggest you schedule the scan for one hard disk, let that run, and then read the report to see how long the scan took (put in some extra time to be sure).

This is a feature that's on the list for future versions, but there are other priorities for now.