Ok I begin to see what you are doing although I have not used this system myself so familarity is not the word of the day.
I assume each shop has it's own database?
I assume that each "shop" will be located by it's own www.xxx.com address?
That being the case I'd install PS3 in the root as I said before and get it working. Then I'd leave that one in place as a functional test working shop.
Then start installing in the sub directories one shop at a time.
My only concern ( and I'm no expert) is that PS3 uses php to assign the root directory which is used in all the files to set the path.
What I'm wondering is if your root is \ and your subdirectory is sb - but if you follow this is actually the root for that shop should you be using just \ if that is the root.
Sorry I'm confusing myself. In a nut shell is your perceivable root set by you webserver to already be your sub directory? or does it see it asa path from the real root on the web server.
Let us know how you get on