How to use Time Machine with multiple internal disks to a single external hard drive

I have a MacPro desktop computer (mid 2012) with 3 drives:

A. 500 GB Flash Storage - used for programs

B. 2 TB SATA Disk - used for Data

C. 2 TB SATA Disk - used for back up of Data disk

When I set up the computer initially, I chose to use 'C' as my back up disk for Time Machine, and only set it up to back up 'B'.

I am going to purchase a 5 TB La Cie external HD and want to set it up to back up all 3 above disks, and subsequently use disk 'C' above for regular use.

My questions:

1. How do I remove the backup data from 'C' above, prior to backing up to my new external HD?
2. What do I need to change in Time Machine in order for it to back up all 3 drives?

Thanks in advance.