I own copies of all three operating systems and would like to have a single drive that enable me to boot from and use Disk Warrior etc under each OS. Is this possible?
Use Disk Utility to partition your external drive into three volumes. This kbase article can help you (use instructions for a startup disk). Name the volumes something that will help you identify them when you option-boot into them later (something like "Tech 10.6", "Tech 10.7", "Tech 10.8"). I recommend at least 8-16GB for each partition to allow the OS to install and room for additional software.
Then follow your normal procedure for creating your bootable Disk Warrior environment. If you are installing OS X 10.6, 10.7, 10.8 respectively onto each volume, just use the typical OS X installation instructions specific to each OS. Ensure that you are on a Mac capable of installing each OS and ensure you select the correct volume from the external drive.
When all is said and done you will be able to option-boot into each environment as long as the machine hardware/firmware is capable of booting into the respective OS you select.