Redhat has a graphical partitioning tool as part of the installer. You should be able to install Linux on the third partition with no problem. Try Here
At a minimum, you will need to delete on NTFS partition, and then make a Swap partition (2x your RAM) and make the rest ext3 for the root partition. Linux will install a boot loader, that will allow you to boot either Windows of Linux.
However, Redhat 7 is now ancient history - it is about 4 or 5 years old. I Strongly
recommend getting a newer version of Linux, such as Fedora Core 4, or you could try Ubuntu which is better for newbies.
I hope this helps