After some discussion on another machining forum, I created a web-based calculator for calculating the diameter of a bore based on the diameters of three gage pins (or any other cylinder) that fit into it.
It's free, open source software that I built as a side project in my spare time. I wanted to show it to the CNCZone community, hopefully it's of use to someone!

You can access it here: https://www.holecalc.com

Let me know if you have any feedback or issues!