3 Pro is quickly approaching one year old. There's almost no reason to benchmark it, especially since a new version of Gemini Pro was supposed to be released mid 2026 and hasn't seen the light of day.
It's not a technical problem, it's a commercial one. If Google can't ship a model to replace the one they deprecated, that tells you everything you need to know about choosing a Gemini model for whatever you're trying to do.
That link shows 3.1 pro listed as deprecated with no replacement model.
Just like 3.7 Flash, 3.1 Pro says "No shutdown date announced."