How to create third partition after Macintosh HD and Boot Camp partitions?
Solution 1:


Shrink the Mac partition.
Create TWO partitions in the remaining space.
Set up windows in one.
Set up data in the other.


While I have not run the windows 7/8 installs, the older versions would present an interface to use an existing partition if present.  If not present, it would default to using the entire remainder of the disk, but this could be over-ridden.  So you may have an option in an ordinary install to not use the entire remainder of the disk.

Solution 2:

Use an external drive as your Data drive.  Using a cloud drive such as DropBox, or Google Drive is also a possibility.

Opinion warning:
I'm not a fan of multi-boot.  The stuff you need to do is always in the OS that isn't running.  Many sad tales of installation processes that were unkind to foreign OSs.  Now that both Parallels and Virtual Box have reasonable performance, I consider that for most uses, a virtual machine is a better solution. 

So...

Solution 3.

Run windows in a virtual machine, and either export a file system via Samba (SMB) or by the virtual environment's file sharing system.  This alternative can be expanded to allow DOS VMs Linux VMs, Solaris VMs.  Note that screen handling can be kludgy for any OS that doesn't have the appropriate extensions.

Solution 4. 

Run Windows on a separate box and connect to it via Remote Desktop.  Again, clunky.  But I know guys who use their iPad this way to connect to their windows box at work.  Not sure how much serious work they get done this way, but fetching the power point presentation they left at work may be sufficient.

Solution 5.

Run windows on a machine next to your Mac so that you can spin your chair.



Bootcamp makes the most sense if you actually need to use the same hardware at maximum performance.  But unless you are needing access to massive amounts of data from both OSs an external hard drive or even an 8 Gb thumb drive is likely addequate.

If performance isn't as critical, then running in a VM gives you access to both environments, and the additional security of having a mac beteen your windows box and the cruel world.  (Yes macs can be pawned, but it now means that an attacker has to get through 2 OSs defences.)

The Remote Desktop solutions require a reasonably high speed data link.  

The 2nd computer solution is less than ideal for portability. 

Hope this helps.