Not sure if Boot camp will let you make the third partition or not. One thing you might try if you still have partition magic would be to create 2 partitions with boot camp, an OSX partition and a Windows/other stuff partition. You can then boot into windows and use partition magic to divide C: into two partitions, one for Windows and the other for the third partition you want. If you want both OSX and windows to see this partition then you will need to format it as FAT32. While this work around should work make certain to back everything up first (you do have an external back up drive, right? If not I highly recommend getting one as your first move.)
Flopticalcube's suggestion of getting drivegenius may be the better option though (still recommend the getting the external BU first)