So, the lyrics were written, at the latest, in 1973 (if they’re the same as in the original studio version, which they seem to be). What would be the reason for including the pope- and wall-references at that time? Perhaps the ‘wall’ line is a genuine prediction on Stewart’s part, but I imagine the ‘pope’ line is just an off-handed attempt at suggestiveness, like the ‘two great men’ one (unless anyone can spot an obvious target for the latter for me), that happened to fall unhappily close to the truth.
Using Wikipedia, I can’t see any recent popes with ‘Paul’ as a birth or regnal name until Paul VI, the incumbent at the time the song was written. Perhaps that was reason enough to make the reference – after all, he had to die at some point, and for obvious reasons he was spending a lot of time near Rome. (On the accuracy of the prediction as sung, Wikipedia says Paul VI died about 15 miles from Rome, which is a bit more than 3 leagues – between about 4 and about 8, depending on whose league you’re using. The two subsequent (John) Pauls died closer than that, though. If you count a John Paul as half a Paul, then perhaps when you tot up the distances you can get 2 (1+0.5+0.5) Pauls dying a net distance of 6 leagues from the gates of Rome. Or if you count a John Paul as a full Paul, but use smaller leagues, you can get 3 dying a net 9 leagues from the gates of Rome. So there was a significant period of time over which the average distance of pope deaths from Rome was 3 leagues per Paul; when you also have to worry about rhyme, it seems quite reasonable to express that by saying: over this period, for a Paul, the distance of their death from the gates of Rome is 3 leagues.)