Benvolio is actually not present in Act 5, Scene 3. The last scene in which we see Benvolio is the moment when he explains to Prince Capulet how it came about that Tybalt slew Mercutio and Romeo slew Tybalt in Act 3, Scene 1. However, we can deduce what Benvolio's perspective of the final tragedy might be and whom he thinks is blameworthy based on Shakespeare's characterization of Benvolio in the earlier scenes.
One thing we know about Benvolio is that he has an excellent sense of justice. We especially see his sense of justice when he leaves out no details in his explanation to Prince Escalus of Tybalt's and Mercutio's deaths in Act 3, Scene 1. Benvolio makes a very specific point of informing the prince that Romeo tried to stop the fight between Tybalt and Mercutio, as we see in his lines:
Romeo, that spoke him fair, bid him bethink
How nice the quarrel was, and urg'd withal
Your high displeasure. (157-59)
Not only that, he makes a point of describing Romeo's general attitude at the moment to establish Romeo's character, as we see in Benvolio's lines, "All this, uttered / With gentle breath, calm look, knees humbly bowed" (159-60). Benvolio's efforts to explain things so thoroughly and to establish Romeo's character shows us that Benvolio believes Romeo is not ultimately to blame for Tybalt's death, but rather Tybalt's own temper is to blame. However, Benvolio also makes a point of explaining that after Tybalt killed Mercutio, Romeo became filled with desire for revenge and fought Tybalt, killing him, before Benvolio could act and separate them. Therefore, Benvolio even rightly sees that Romeo's unlawful desires for revenge are also responsible for Tybalt's death. Hence, while Tybalt is more responsible, Benvolio sees that Romeo is not without fault.
We can deduce from this that Benvolio has a very strong sense of justice and right and wrong. After hearing Friar Laurence's account of what happened and his involvement in the deaths in the final scene, Benvolio might have felt that Friar Laurence made some poor decisions that helped escalate matters leading towards the deaths; however, Bevolio would ultimately agree with Prince Escalus in believing that it is Lords Capulet and Montague who are ultimately responsible for the deaths in the play due to their hatred and their feud. Therefore, Benvolio's perspective on the things that took place in the final scene, as well as in the play as a whole, would most likely agree with Prince Escalus's perspective that Lords Capulet and Montague are to blame.