The developer of Spire (chpwn) has pretty much decided he's not going to try to get Spire to work on iOS 5.1.1 (which then means the iPad3). Evidently, there are legal issues and the workaround is too much of a hassle (or impossible). According to his tweet, "it's 'impossible' to update Spire to support iOS 5.1: Apple does not host a decrypted copy of Siri for iOS 5.1." He has further clarified that there he has absolutely no way of porting Siri to unsupported devices in iOS 5.1 (this means/includes 5.1.1 also).
The long and short of it is that, AFAIK, there is no way to get Siri on an iPad3 running iOS 5.1.1. No such hack or tweak exists or if one does, it's a kludge that won't work.
Marilyn
P.S. As you talk about putting Siri on the iPad3, I'm going to assume you have jail broken your iPad. Therefore, I've taken the liberties of moving your post over to the Hacking section. Thanks for your understanding.