You can access the partition folders from
/Volumes (as mentioned below). To create the folder, you can do so by creating an Automator Folder Action such as this one:
Create a new Automator Folder Action and set the path to the Applications folder of the Volume. Then, drag the "Get Folder Contents" action in, and check "Repeat for each subfolder found." Then, drag in a "Copy Finder Items" action, and modify the path to your choosing. Check "Replace existing files" so that if you have duplicate applications in each of your /Applications folder you won't get an error. NOTE: this will override any application data.
I created this one, but all you should have to do is to clone it three times (for each of the partition folders) and then modify the paths. You can also change whether the apps should overwrite other apps. I hope that this answers the question better than my other answer! I'll see if I can come up with some better solutions. Also, I don't think that you will be able to filter the apps based on the OS X version.