Can a novel as complex as Pride and Prejudice be summarized in three sentences?
Let's be clear in that, when we summarize in a very short synopsis the contents of a novel that in some edition reaches 71 chapters, we need to limit our summary to the very basics of what happened at the beginning, middle and end. This is true especially if we are to use only three sentences to do this. Also remember that there are no specific rules as to how long a sentence is supposed to be, so let us assume that we can use complex and compound sentences as one. These compound sentences have more than one verb and sometimes express more than one idea. That may be one way to get away with only using three.
One possibility about what happens at the beginning may be:
"A rich bachelor moves into a town and the marriageable humbler girls of the Bennet family befriend him and his friend, creating two potential love connections."
The middle has to be compressed as best as possible to go straight to the gist of it all:
"Misunderstandings caused by class and social differences ruin the chances of the Bennet sisters and leave the main heroine bitterly questioning issues the pride of social superiority, and the prejudice of classicism."
The end will have to also be compressed as best as possible, and may look like this.
"Elizabeth discovers her wrong perception of Darcy and her own pride and prejudice against him; the reality of each character comes to light and the two main couples marry and live happily together pushing all prejudices aside."
Again- this example parts from the premise that you are writing complex and compound sentences. Be sure that, when you write your sentences, you eliminate details and, instead, move the plot forward as fast as you can.