Triple Booting Question

If you can add another partition via Disk Utility, then yes, you can install Mac OS X 10.6 Snow Leopard via the grey Restore DVD that came with your Mac.
If you cannot add another partition, then that is due to data being stored there and you need to clone the entire HDD with its two partitions to another, properly formatted, HDD with two partitions via CarbonCopyCloner or SuperDuper!, delete the old HDD and partition it anew.

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Links to guides on how to use Disk Utility, the application Mac OS X provides for managing internal and external HDD/SSDs and its formats.

Guide on how to use Disk Utility from the restore / installation DVD.

Using Disk Utility - Formatting, Partitioning, Verifying, and Repairing Disks

which includes:

• Format, Erase, or Reformat a Drive
• Erase or Reformat a Partition
• Add a Partition
• Deleting and Resizing Partitions
• Disk Formats

and some more guides
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DiskWiping – One Pass is Enough​

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How to format a new HDD to install Mac OS X onto (or make a bootable copy onto)​