How do the actions of Benvolio, Tybalt, and Mercutio in Act III, scene I express their characteristics as they were presented earlier in the play?
Benvolio previously established himself as a proponent of peace in I.i, when he asked Tybalt to help him "part these men with me" (I.i.64). Tybalt established himself as Benvolio's foil, a violent and aggressive character, answering "I hate the word [peace]/ As I hate hell, all Montagues, and thee" (65-66).
Mercutio was characterized as a good-natured, excessive talker, prone to long dissertations on the nature of life and love (Queen Mab speech, i.e.). He is also a teaser and instigator, as seen in Act II, sc. iv.
These characteristics are all similarly conveyed in III.i.
Benvolio: I pray thee... let's retire. The day is hot, the Capels are abroad, And if we meet, we shall not 'scape a brawl.... (1-3).
Mercutio then speaks for 13 lines about how much of a quarreler Benvolio is, and they have a humorous back-and-forth until Tybalt, aggressive as ever, interrupts.
Tybalt's first words to Romeo: "...thou art a villian." (59).
Those are some fightin' words in late 14th century Verona!
Mercutio also speaks similarly to Tybalt as he did to the nurse in II.iv, but is much more insulting and without all the sexual innuendo.