Perhaps scan the folders and see if you find three files with the same primary file name, but different ending variants of the file name. For example, perhaps your project is named MyProject. In that case you might see:
MyProject.HTM (main landing page)
Incidentally, your situation underscores something I toss out for each RoboHelp HTML class I facilitate. Always consider naming the start page index.htm. This has some advantages.
Helpful and Handy Links
Yep, I've chosen "default.htm" (rather than index.htm)--no clue as to logic in these legacy help projects, tho, other than neither of those were chosen.
In the whskin_info.htm file, the file you want is identified as the "Start Page."