This is a quick video on how to create a bootable hard drive and install 3 Operating Systems. For this video we are going to install 10.10 Yosemite, 10.11 El Capitan, 10.12 macOS Sierra. You must create a bootable USB of each of the Operating system before you reboot the machine to run the installer. Here is a link to the website to help you do this.
http://macuser.org.uk/2017/02/02/command-line-to-create-usb-os-stick/Once you have created your USB Bootable OSX drive reboot the machine you will use to install the OS to the external drive. YOU MUST PRESS THE ALT KEY ON THE REBOOT. This will open up the bootable options for the machine. Select the USB bootable installer you created and run through the installer selecting the Ext hard drive for that OSX as seen in the early part of this video i.e Yosemite 1010. Once the OSX is installed create use for the new OSX you just installed on that drive and run the updates. Repeat the process for all other OSX you want to install.
Remember only certain machines will boot certain OSX systems. In short anything under 2010 will not install macOS Sierra (there are a few exceptions)
Here is the video on how to create a USB installer:
https://www.youtube.com/watch?v=FOjyy5L23R0&feature=youtu.be