Fate is the ultimate variable; we could argue that the young lovers are "fated" for disaster. Their feuding families have a long history; both are young and hormonally influenced; neither has a strong parent to advise them; the priest they trust is inept at best, evil at worst.
Chance and fate are not easily separated. But by chance, Juliet bumps into Romeo who has crashed her parents' party. Chance places Mercutio in the path of his, and Romeo's, rival; chance, or arguably, fate, causes the letter of explanation to Romeo regarding Friar Laurence's sleeping potion scheme to remain unknown.
Choice is more easily understood. The lovers make a clear choice to defy their parents' wishes. The friar makes a choice to be strong-armed into Juliet's will (she threatens suicide); the Nurse also chooses to aid and abet the couple. The list goes on and on.
Quotes that illustrate "fate", chance", and "choice":
Fate: My only love, sprung from my only hate! / Too early seen unknown, and known too late! / Prodigious birth of love it is to me / That I must love a loathed enemy (I.5).
Chocie: Deny thy father and refuse thy name. / Or if thou wilt not, be but sworn my love / And I'll no longer be a Capulet (2.2).
Chance: But soft! What light through yonder window breaks?
It is the East, and Juliet is the sun! (2.2)