How do the actions of Benvolio, Tybalt, and Mercutio in scene 1 of Act 3 express their characteristics as they were presented earlier in the play?
Most authors give characters names which tell something about the characters themselves, but even when they don't, the characters' actions give away their true personalities.
Benvolio's name includes the root "bene" which means "good". True to his name, Benvolio acts as the peace-maker and mediator to all his friends. When Romeo is upset about Rosaline, Benvolio attempts to pick up Romeo's spirits and help him get over her. It is Benvolio who suggests that Romeo go to the Capulets' party to see (and fall in love with) other pretty girls. Of course, this is where Romeo meets Juliet. Benvolio is the one who attempts to keep Mercutio restrained a little, and he alone seems to understand how the heat is effecting everyone on the fatal day that takes Mercutio's and Tybalt's lives.
Mercutio, another of Romeo's friends, is moody and unpredictable--rather like mercury when an old thermometer is broken...the little silver balls of the element roll all over the place and in end up in the most peculiar spots. This character is upbeat and fun one minute, and depressing the next. Mercutio is witty and sophisticated as indicated by his speeches, and being a relative of the prince and friends with Romeo and other upperclassmen, we would expect him to be educated. He speaks often of sex, quarrelling, and strange exotic people and places making him a good character for comic relief. However, he loses his life because of his inability to back up and rethink after he lets his mouth run wild. He is a dramatic sort, and his death is no less dramatic than the rest of his life--in fact, before dying, his moodiness strikes again, and he puts a curse on both the houses of Montague and Capulet, blaming them both for his demise without taking any of the credit for his situation.
Tybalt, the only member of the house of Capulet in your inquiry, is also hot-headed and stubborn. The clipped sound of his two-syllable name just sound like sword fighting. He is ready to fight at any minute, and he takes his role as a Capulet in the life-long fight against the Montagues seriously. When he recognizes Romeo at his uncle's party, he approaches his uncle to reveal young Romeo's identity. The uncle is willing to overlook the boy's presence, but Tybalt wants to fight and kick Romeo out. He is more than just a little upset when he is told to leave Romeo alone. Tybalt smolders in a corner afterward, and vows to get revenge. The next day, he sends a letter of challenge to Romeo's house. Of course, his challenge and insults incorporate all of Romeo's friends. Tybalt wants Romeo to pick up his sword and fight, but Romeo will not since he is now in love with Tybalt's cousin, Juliet. Mercutio instead takes on the challenge, and while Romeo tries to hold Mercutio back, Tybalt's skillful sword finds Mercutio under Romeo's arm. Tybalt then opens his big mouth to declare Romeo will soon follow Mercutio to the grave. However, it is Romeo who wins the battle, and Tybalt is slain out of rage and revenge for Romeo's friend, Mercutio.