Are there "love triangles" in Pride and Prejudice by Jane Austen? If so, what characters are involved in them?
There are actually several "love triangles" in Pride and Prejudice by Jane Austen, although they do not dominate the plot as many of them are only mentioned second-hand.
In the major romance of the book, that between Elizabeth Bennet and Fitzwilliam Darcy, there is in fact a triangle that affects the plot, and especially the motivations of one of the villains of the book. Lady Catherine intends that Darcy marry Anne de Bourgh, and thus resents Elizabeth.
During the period that Darcy is silently in love with Elizabeth, and Elizabeth intrigued by Darcy but still disdainful of him, the oleaginous clergyman William Collins proposes to Elizabeth. When she refuses, he successfully proposes to her friend Charlotte Lucas.
Mr. Bingley's sister Caroline wishes to marry Mr. Darcy, although he has no actual interest in her. This creates and Elizabeth-Caroline-Darcy triangle. It also motivates Caroline to be unpleasant to the Bennet family.
Both of Mr. Bingley's sisters wish him to marry Georgiana Darcy, although he isn't particularly interested in the scheme. That would create a potential Georgiana-Jane-Bingley triangle.
Wickham has eloped with Georgiana, flirted with Elizabeth, and eventually elopes with and marries Lydia.
There are a couple love triangles in Pride and Prejudice. They each involve Mr. Darcy and Elizabeth Bennet.
Elizabeth is interested in Wickham, but Darcy is in love with Elizabeth. Darcy is clearly no fan of Wickham's; Wickham frames a story from his past to make Darcy look careless and awful. Wickham is actually the careless one. He once tried to elope with Darcy's young sister so that he could have her inheritance. Darcy caught them before anything could happen.
This triangle is settled when Wickham elopes with Elizabeth's youngest sister, Lydia. The story comes to a head when Elizabeth finds out that Darcy paid Wickham to marry Lydia—ensuring no shame would be brought to the Bennets. Elizabeth was clearly already falling in love with Darcy during this time.
Another less important love triangle is Darcy, Elizabeth, and Anne de Bourgh. Anne and Darcy were betrothed as children; their families wished for them to be wed. When they grew older, they chose not to marry. Despite the wishes of Anne's mother, Lady Catherine, Elizabeth and Darcy fall in love. Though Lady Catherine tries to coerce Elizabeth into promising to never become engaged to Darcy, she fails to do so.
Lady Catherine walks into the gardens to talk to Elizabeth alone and says:
"A report of a most alarming nature reached me two days ago. I was told that not only your sister was on the point of being most advantageously married, but that you, that Miss Elizabeth Bennet, would, in all likelihood, be soon afterwards united to my nephew, my own nephew, Mr. Darcy. Though I KNOW it must be a scandalous falsehood, though I would not injure him so much as to suppose the truth of it possible, I instantly resolved on setting off for this place, that I might make my sentiments known to you."
She goes on to tell Elizabeth that Anne and Darcy are engaged. When Elizabeth says that if they're engaged, Lady Catherine doesn't have to worry about him marrying Elizabeth, Lady Catherine explains:
"The engagement between them is of a peculiar kind. From their infancy, they have been intended for each other. It was the favourite wish of HIS mother, as well as of her’s. While in their cradles, we planned the union: and now, at the moment when the wishes of both sisters would be accomplished in their marriage, to be prevented by a young woman of inferior birth, of no importance in the world, and wholly unallied to the family! Do you pay no regard to the wishes of his friends? To his tacit engagement with Miss de Bourgh? Are you lost to every feeling of propriety and delicacy? Have you not heard me say that from his earliest hours he was destined for his cousin?"
Elizabeth stands up for herself, though. In the end, she does indeed marry Darcy.