"3 seems rare and might have struck those who passed through that this was a special place and worth settling. The midpoint of that key geo-allignment being St Pauls, perhaps explaining why it became so important."
I really think you're onto something there. I've heard St Pauls has a pre-Christian temple underneath it? What you say reminds me of a standing stone near Edinburgh (don't ask me its name) that stands pretty much exactly half way between the two most eye-catching hills in the area.