Often, the heroine ends up with the 'right' man. For example, in many cases, the man who truly understands her and has a deeper connection wins her heart in the end. So, if there's a rakish character involved in the love triangle, she usually chooses the more honorable and loving one.
Often, the protagonist makes a clear choice between the two love interests. For example, in some novels, the girl might realize that one guy has been there for her all along and truly understands her, so she chooses him over the other.
Often, one person in the triangle is left heartbroken. For example, in 'Romeo and Juliet', Paris is left out as Romeo and Juliet pursue their love and ultimately die together. In some cases, the main character realizes who they truly love and choose that person. Like in 'Pride and Prejudice' Elizabeth chooses Darcy in the end.
Often, one person makes a choice. For example, in some stories, the female lead chooses the guy who she realizes truly loves her and has her best interests at heart.
Often, one person in the love triangle ends up with the main character they are meant to be with. For example, in 'Inuyasha', Inuyasha finally chooses Kagome over Kikyo. Another way is that the character in the middle realizes they don't have romantic feelings for either of the other two in the triangle and decides to be alone or just be friends, like in some less - common storylines.
They often end with one person making a choice. For example, the person at the center of the triangle decides to be with one of the suitors. Or it could end with all parties realizing it's too complicated and walking away.
They can end with one person making a selfless choice. For example, one of the people in the triangle realizes that their friend really loves the person more and steps aside. Another way is that the person who is the center of the triangle chooses one of the two. Sometimes it's a happy ending where the couple stays together, and the other person moves on.
One popular one is 'Pride and Prejudice' by Jane Austen. It has a sort of love triangle between Elizabeth, Mr. Darcy, and Mr. Wickham. Elizabeth is initially attracted to Wickham, but as the story unfolds, she realizes her true feelings for Darcy.
Some end with the unrequired love becoming mutual. For example, in many contemporary romance novels, the two characters finally realize their feelings for each other and get together.
Often they end happily. The lovers overcome the obstacles that made their love a secret in the first place. For example, in 'Pride and Prejudice', Elizabeth and Mr. Darcy get married after all misunderstandings are cleared.
In some novels, the object of the unrequited love might have a change of heart at the very end. This could be due to a realization of the depth of the other person's feelings, or some life - changing event that makes them see things differently. However, this is not always the case. There are also endings where the character with unrequited love accepts their situation and learns to love from a distance, finding a sort of peace in just having the other person in their life in some way, even if it's not a romantic relationship.