Just a little clarification:
The file called IPSW is the firmware (the iOS). You can download it whenever you wish; any version number. The problem comes in when you try to install it. Apple only authorizes the installation of the most current IPSW/iOS. Today, that is iOS 6.0.1.
SHSH blobs are not the IPSW, but you need them to use jail breaking tools to downgrade, restore or re-restore an iOS version. See here for all you ever wanted to know about SHSH blobs (:)):
http://www.ipadforums.net/showthread.php?t=59291
It is these SHSH blobs that are specific to a device and are not interchangeable amongst devices. They cannot be shared.
However, none of this matters in your case. Whether you have the SHSH blobs or not - there is currently no way to downgrade an iPad3 running iOS 6.x to iOS 5.1.1. None. Once you moved to iOS 6.x, there was no going back.
Hope this clarifies.
Marilyn