Your're looking at the major release page.
Go up one level on the software download tree:
https://software.cisco.com/download/type.html?mdfid=286304360&flowid=79748
and look under Prime Infrastructure patches:
https://software.cisco.com/download/release.html?mdfid=286304360&flowid=79748&softwareid=284272933&release=3.1.1&relind=AVAILABLE&rellifecycle=&reltype=latest
You do need to have 3.1 installed prior to the 3.1.1. patch.