I've heard this notion many times, that par 3s are effective "gap fillers." Perhaps.
Stanley Thompson was one golf architect who saw par 3s in a different light. He routed his golf courses by, first, identifying the ideal natural locations for one-shotters, then essentially, using the par 4s and 5s as "gap fillers."
Studying Thompson's courses reveals his methodology to undoubtedly be true. His one-shot holes are consistenly in the best natural locations on a given property, and thus always the highlight of his courses. Always. And, another interesting note is, many of his courses feature five par 3s.
My theory is, Thompson pirated this method of routing from Harry Colt, who reportedly, also routed his courses in a similar manner. Thompson was a young caddie at Toronto GC when Colt came over in 1911 to design a new course for the club. And, two years later, when Colt returned to Canada to lay out a new course for Hamilton G&CC, Thompson's older brother, Nicol, was the pro there. Young Stanley surely observed the master, Colt, at work on both projects. That's my logical presumption anyway.