If you install each OS independently, making sure it does not see an "active" partition (except the empty one you are installing it to), then each will have a vanilla set of boot files for that OS alone.
You can then boot each system by making the OS partition "active" and rebooting.
When each OS is on its own HDD, you can leave them all active and boot the one of your choice by overriding the BIOS boot device.
You also have the option of multi-booting by subsequently adding additional entries in the boot menu of whichever OS you deem to be "in control", chaining to each of the other OSs.