In order to unify the three topics of the body paragraphs, you will want to tie these ideas to the one larger idea of fateful impetuousness that the "violent delights" of Romeo and Juliet effect. This unifying idea is, then, the thesis of the essay.
For example, you could write that the aura of fateful impetuousness leads to the rash actions of the Montagues and the Capulets, "the violent delights" of Romeo and Juliet, and the poor decisions of the friar.
- The Montagues and the Capulets
Lord Capulet proves himself impetuous in his actions early in the play as he requests his sword when he observes the fray between several members of both the Montague and his house. Then, in Act IV, after having advised Paris to wait for the hand of his thirteen-year-old daughter, he hastily decides that Juliet should indeed marry Paris in order to comfort her in her grief for her beloved cousin Tybalt, whom a Montague has slain.
Despite Juliet's cautious warnings in the orchard, telling Romeo not to swear by the inconstant moon, and fearful that their "contract" is "too rash, too unadvised, too sudden," she and Romeo marry after just meeting one another, despite the friar's advisory caution, "Wisely and slow. They stumble that run fast" (2.3). Then, Romeo endangers this marriage through his rage and reckless behavior of attacking and killing Tybalt after his friend Mercutio is slain. Further, he acts hastily as he rushes to Juliet's grave to ascertain if she is dead as he has heard. With fatefully impetuous behavior, Romeo slays Paris who comes to find Juliet, as well. Then, tragically, when he sees Juliet, whom he assumes is dead, he impulsively takes poison.
Because he has agreed to perform the marriage ceremony of Romeo and Juliet, Friar Laurence feels responsibility for the young couple. So, when the frantic Juliet comes to him, he reacts with haste, feeling the forces of fate against Juliet and her Romeo and, perhaps, even him. He then prescribes a potion to Juliet to make her appear dead, deceiving her parents so that they will be overjoyed when she awakens and then forgive her for deceiving them. He also sends a letter to Romeo, advising him; however, a plague causes the town to be quarantined and Balthasar cannot reach Romeo with the friar's message. Consequently, the friar nervously goes to the catacombs to attend Juliet when she awakens. But, fearing the guards, he rushes out and leaves Juliet to discover her dead Romeo.
Who, then, is to blame for the deaths of Romeo and Juliet? Perhaps all contributed to the fate of these lovers, but in the final analysis it seems Friar Laurence, who acts in loco parentis for Juliet and Romeo, marrying them and taking a private role in their lives, is the most responsible for their tragic fates.