Hi
Looking at the size of the drives I would guess that this is one 500 Gig hard drive partitioned into two drives.
And that the 3rd partition is the recovery partition.
The reason for doing this is generally so that if you have a failure of Windows or a virus of some sort that requires re-installing the operating system completely you can do so without losing all you files.
With the hard drive partitioned you can format drive C:\ and leave everything on Drive D:\ untouched.
I always did this in the old days when I had only one hard drive.
Now I always have multiple drives and I don't have anything except software and basic Widows stuff on my C:\ drive.
I have that all backed up to another drive so it's not worth partitioning...