You should be able to accomplish this without too much effort. Let's assume you have Windows installed and working using the entire drive.
First, defrag your Windows install at least twice. Then shrink the Windows partition to whatever you feel is appropriate, so you will have space for BSD and Linux Mint. Since you are probably familiar with the Windows tools, use them to shrink your main Windows partition.
Next use the Mint CD and open Gparted, it is in the applications menu. You should see the space you liberated from the Windows partition as unallocated. Make the entire unallocated space into an extended partition. Then make three logical partitions in the extended partition. One for Mint, ext4. One for your swap, swap. And one for BSD, whatever FS you want/need to use for it.
Install your BSD next into the pre-made partition. Be sure to install grub to the correct partition in the installer.
Then install Mint 8 into it's pre-made partition. You will need to use the manual partitioning mode for this. Again, be sure to assign grub to the correct partition on the last installer screen before the actual install. The button is called "advanced" I think.
Now, here is the key. After you have installed Mint and before you reboot, open Gparted again and set the boot flag/active partition to the partition Mint is on.
When you reboot you should get the grub menu where you can select the system you want to boot.
If you wish to go back to Windows only you would boot the Mint cd again, open gparted and change the boot flag back to the Windows partition.
Hope this works out for you. Incidentally, we have "viking777" to thank for getting me started looking into this method.