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...