I'm assuming you mean tropes, and it's because the original story tellers used three, leading to story tellers after that to copy them and use the rule of three, and so it just became a normal thing.
I would say that King would want to discuss the Viet Nam war with his "fellow Americans and not Hanoi or the NLF because they were totally capable of fighting the US and it was internal opposition from within the US which he was concerned with to voice strong disapproval with American aggression in Viet Nam so his "fellow Americans" were who he could have influence over.
Come in March is the prepositional phrase
it is also adjective phrase
Within these lines we see simile/metaphor, which is a
literary technique of making a comparison between two things that are generally
not considered similar in an effort to give readers a better understanding of
the topic being discussed. In the lines
we can see that the sound bees and flies make is being compared to the sound a
lute (which is a type of guitar) in order to give readers who might be familiar
with the sound of the lute (and unfamiliar with the sounds of nature) a basis
for comparison in the understanding of how the sound bees and flies
sounded. As such, what these lines are
saying is that the sound flies and bees made while they ate fruit sounded like
the music made by a lute.