When I installed the HOMM3 save game editor from Github I was able to open scenarios and pick a hero. But when I did so, the place where the tabs and info showing options is blank. That is the page just shows the options for File or Open and also the row for selecting a hero or searching. Below that, there is nothing at all.

What is happening? How do I use this program? I'm on Win 10

Yep, is not really straightforward... selecting the hero from the combobox isn't enough, you also have to always hit the search icon at least once, then it should load all the parameters. You can repeat this for multiple heroes, they will load in a separate tab

By the way, you mentioned elsewhere that you have the game on Steam... note that if you mean the HD Remake version (which is entirely different package than having H3 Complete from GOG and the HD Mod), savefiles may not be 100% compatible with the editor