I'm not that familiar with Flynn's works in Phila., but it seems to me, those architects with an awesome inventory of par 3 holes (Stanley Thompson for example), were very good at utilizing the very best natural sites for those holes. Thompson claimed to identify those sites first, then route the remaining holes around the par 3s.
I think a majority of golf architects, past and present, would agree that par 5s are probably the most difficult holes to make really interesting.