prepare the third drive by disconnecting drives 1 and 2 and installing Kali on the third drive. once the OS is on drive 3 and everything therein is working, remove the drive and reinstall drives 1 and 2 and verify that they boot as they should. when you are sure the drives are good, add the third device. verify the boot order sets it as the last to boot. I assume you will be booting into the existing OS as the default.
now you will use the boot device selector on the motherboard to manually boot kali. kali will be there but you will need to trigger its boot by telling the motherboard to boot to it when you need it.
this is how I am running my system. 10, 7, mint
Canned Grounding Rant-
shut down system and remove side panel. with the power cable plugged into the PSU touch a bare unpainted metal area of the case. (my favorite spot is an unpainted screw securing the PSU) once you have grounded yourself you can unplug the computers power cable from the PSU and can touch the system.
if you move your feet, or shuffle in your chair, plug in the cord, reground yourself and unplug again.
end canned rant-
never remove a hard drive while the system is powered on, shut down the system when needed