Only the latest version of the toolkit is available for download - because the toolkit always maintains (theoretically) perfect backwards compatibility. Report of any break in backwards compatibility would be treated as a bug report by the developers.
Is there any particular reason you are looking for v3.1.0?
Thank you for the quick response. My client has strict guidelines on what software is installled, so we can't simply upgraded to the the current version. In short, v3.1.0 is an approved version for this client.