"The Citizens are up and Tybalt is slain. Stand not amazed. The Prince will doom thee death if thou art taken. Hence gone, be away!" (3.1.139)
Romeo Blames Luck to be the reason why he killed Tybalt
"O, I am Fortune's Fool!" (3.1.142)
Fate and luck are used in this scene due to Romeo blaming it for the reason why he killed Tybalt. It also shows how bad luck and fate are tied to Romeo as this ends in him being banished from Verona.
This relates to the motif of fate as due to Romeo going to the party, Tybalt requests to fight him which ends in the death of Mercutio and then leads to Romeo killing Tybalt after.
The quote "Fortune's Fool" that Romeo says means that he has fallen victim to bad luck and is a fool for doing so. This proves to be even more bad luck for Romeo as it ultimately ends in him being banished from Verona which leads to hime being separated from Juliet which sets up the final two acts where to two "star-crossed" lovers are supposed to die.