This is from my son's college book called The Art of Oratory Handbook
It is practically impossible to discuss every way in which a speech is written,
because every orator will organize his or her content in diverse ways that
have been proven effective. One constant in speeches, as elsewhere,
is what has been called the "Rule of Three".
There is something that resonates deeply with human nature when three
related items are strung together. If someone makes a sound, a noise,
then that could mean anything and nothing. If that sound is repeated,
it could be a simple accident. Three correlated sounds, though, reflect human
intention – as the standard knock at the door will be three raps; three beats
are also the minimum needed for the establishment of rhythm.
The rhetorical world, going back to classical times, is full of lists of three
(in the Greek term, hendiatris - "one through three": a figure of speech in
which one idea is expressed through three words or phrases)
"Sex, drugs, rock 'n roll", "Friends, Romans, countrymen",
"Peace, order, and good government", "of the people, by the people,
for the people", or the 19th century German formula for the best preoccupations
for women, "Kinder, Küche, Kirche" (children, kitchen, and church).
Think of the satisfaction that comes with all of the old children's literature,
and fairy tales, the Three Little Pigs, Cinderella's three wishes, three blind mice.
Think of all of the jokes that you've ever heard where you know that the
punchline will come precisely when the third person acts.
Politicians are acutely aware of the importance of that "third thing" (Atkinson, 1984).
A recent glaring example can be seen in the fumble by Texas governor Rick Perry
in the US Republican primary debates when he promised to eliminate three departments
were he elected president; after identifying Commerce and Education, failed to remember
the third. The discomfort among those on stage and in the audience at this prolonged
failure to complete the set was acute (Terkel, 2011). Perry opted out of the running in short
order as a result of this rhetorical fiasco.
High school students are all familiar with the five-paragraph essay, yet the question is often
asked why such a constraint is put upon them. The reason remains the same: the three points
of support or development are not only a minimum number of discussions to establish a point,
but they are also rhetorically satisfying.
A chair can only take the weight of a person sitting on it when there are at least three legs,
placed equally apart. If two points of a speech are too close together, as if they were legs
on a stool, the thesis will fall flat. To work best with the Rule of Three, 3 key points in a speech
must be equally meaningful (to the audience and speaker alike), distinct from each other in
a balanced way, and equally relevant to the central theme.