Identify three characters who are the cause of their own negative or tragic circumstances in the Shakespeare play Romeo & Juliet. Provide supporting evidence and quotations. Need 3 pieces of evidence and 3 quotations for each of the 3 characters. Please also explain in detail the meaning of quote "The fault...lies not in our stars, but in our selves."

When examining the narrative of Shakespeare's Romeo and Juliet, we can argue that most of the characters are, indeed, responsible for their own undoing or tragic outcome. Let's take a look at the three players who arguably suffer the most because of their own actions:
ROMEO:
Romeo has long been regarded as one of the most emotionally unstable and impulsive characters of the Shakespearean canon. He begins the play with a broken heart—crushed by his unrequited love for the pure young Rosaline. This experience and the reality that Rosaline has taken a vow of chastity has led him to claim that "in that vow / Do I live dead that live to tell it now." This infatuation, however, is quickly overturned when Romeo meets Juliet and proclaims, "Did my heart love till now? forswear it, sight! / For I ne'er saw true beauty till this night." As we know, this situation quickly escalates with Romeo and Juliet beginning a clandestine affair despite the fact that they are clearly ill-suited for each other; Juliet is intended to be married to Paris, and they both come from different warring families. Despite this, they rage against their circumstances. Ultimately, Romeo is responsible for his own fate when he impulsively decides to buy poison from the apothecary after mistakenly hearing of Juliet's death, travels to her tomb, and commits suicide by her body. He takes the poison, proclaiming melodramatically: "O true apothecary! / Thy drugs are quick. Thus with a kiss I die." Had Romeo had the patience and clear-mindedness to wait until he was fully informed of the situation, he would have discovered that Juliet was indeed alive and waiting to escape Verona with him. 
JULIET:
Much like Romeo, Juliet falls prey to her own impulsive behavior. What worsens this experience is a penchant for the dramatic. Juliet first threatens to kill herself in front of Friar Laurence upon learning that Romeo is to be banished and that she must marry Paris: "O, bid me leap, rather than marry Paris, / From off the battlements of yonder tower." Although he is able to talk her off the proverbial ledge, she continues to make rash choices. She becomes embroiled in the ridiculous plan to fake her own death—one which is destroyed by the failure of the messenger to deliver the details of the plan to Romeo on time—and despite the risk involved, participates. Juliet doesn't even know if the potion she is taking is really harmless or not, asking: "What if it be a poison, which the friar / Subtly hath minister'd to have me dead, / Lest in this marriage he should be dishonor'd / Because he married me before to Romeo?" Thus, Romeo kills himself because of this miscommunication, and Juliet—rather than seeing how destructive this relationship is—chooses to follow in his footsteps, pronouncing, "O happy dagger! / This is thy sheath; / there rust, and let me die." 
TYBALT: 
Tybalt is warned time and time again to cool his anger in the play, and yet persistently ignores this advice. In the very first scene, he throws himself into the brawl despite having no need to do so. When he later spots Romeo at the Capulet's ball, he intends to go after the boy; instead, Lord Capulet instructs him: "Content thee, gentle coz, let him alone." Tybalt argues back that he will not endure "such a villain" as a guest, and is once again shut down by Capulet, who insists, "He shall be endured." This hot-headedness ultimately results in Tybalt's untimely death when he once more starts a fight that he never should have begun—this time with Mercutio, who was not even the intended target. Tybalt had wanted to fight Romeo, claiming, "Boy, this shall not excuse the injuries / That thou hast done me; therefore turn and draw." Despite more cautioning to cease the battle, Tybalt winds up killing Mercutio in the duel, which leads to a chaotic attack from Romeo, who is grieving and vengeful. Thus, Tybalt dies because he was too angry and single-minded to see the absurdity in his own behavior. 
 
As for the quote you mentioned: although it may thematically apply to this play, it's actually spoken in Shakespeare's Julius Caesar, not Romeo and Juliet. This quote simply means that we are responsible for our own fates; our destiny is dictated by our words, thoughts, and actions rather than by some mysterious arrangement of the Universe.