There isn't a big difference between these three. "Past men" is not specific. It could mean men from any time in the past. "Men who lived long ago" is a bit more specific because "long ago" makes us think it probably wasn't in the last hundred years. "Ancient men" is the most specific because of the word "ancient", which implies a VERY long time ago, perhaps several thousand years or more.

Hello, Do you have more context by any chance? I would like to help you.