Identify three characters who are the cause of their own negative or tragic circumstances in Shakespeare's play Romeo and Juliet. Provide supporting evidence and quotations. If possible, provide 3 quotations for each of the 3 characters.

The three characters that I would choose for this question are Tybalt, Romeo, and Mercutio.  All three characters have the same negative/tragic circumstance.  They all die. 
I'll start with Mercutio because he dies first, and I'll provide evidence from Act 3, Scene 1.  This scene begins with Benvolio and Mercutio walking around together in the heat.  Benvolio begs Mercutio to call it a day and head home because he knows that the hot weather brings about bad tempers. 

The day is hot; the Capulets, abroad;
And if we meet we shall not ’scape a brawl,
For now, these hot days, is the mad blood stirring.

Mercutio then gives a great monologue about how Benvolio is the guy that normally fights at the slightest provocation.  

Nay, an there were two such, we should have none shortly, for one would kill the other. Thou, why, thou wilt quarrel with a man that hath a hair more or a hair less in his beard than thou hast. Thou wilt quarrel with a man for cracking nuts, having no other reason but because thou hast hazel eyes.

Those lines strike me as odd every time I read this scene because it is Mercutio that chooses to fight Tybalt.  In fact, I feel that Mercutio does more to goad the fight than Tybalt.  From the moment that Tybalt enters the scene, Mercutio talks to him with open contempt and insults.  Tybalt begins the encounter by nicely saying the equivalent of "hello gentlemen."  


Follow me close, for I will speak to them.
Gentlemen, good e'en. A word with one of you.



But notice Mercutio's response.  

And but one word with one of us? Couple it with
something. Make it a word and a blow.

Mercutio is already goading a powerful adversary into a fight.  Tybalt is hot tempered to begin with, and Mercutio is making it worse.  Romeo finally shows up on the scene and refuses to fight Tybalt.  By this point, Tybalt and Mercutio are both up for a fight.  Mercutio chooses to draw his sword and challenge Tybalt.  


O calm dishonourable, vile submission!

Alla stoccata carries it away. (draws his sword)

Tybalt, you ratcatcher, will you walk?



The end result is Mercutio's death.  
 
Tybalt's death shortly follows, and his death is a result of his actions as well.  Had he not killed Mercutio, Romeo would not have sworn vengeance against him.  


Now, Tybalt, take the “villain” back again
That late thou gavest me, for Mercutio’s soul
Is but a little way above our heads,
Staying for thine to keep him company.
Either thou or I, or both, must go with him.



The other reason that Tybalt is at fault for his own tragic death is the fact that he returned to Romeo.  After killing Mercutio, Tybalt successfully escaped the scene of the crime.  


PETRUCHIO
Away, Tybalt.  Exeunt TYBALT, PETRUCHIO, and the other CAPULETS.


Let's also not forget that much earlier in the play, Tybalt vowed to seek out Romeo and punish him for crashing the Capulet banquet.  


Patience perforce with willful choler meeting
Makes my flesh tremble in their different greeting.
I will withdraw, but this intrusion shall
Now seeming sweet, convert to bitterest gall.



If Tybalt had listened to Lord Capulet and just let the infraction go, he never would have gone looking for Romeo.  If he isn't looking for Romeo, then the entire confrontation that ends with his and Mercutio's deaths never happens. 
 
Finally, let's look at Romeo.  Let's also use Act 3, Scene 1 as a starting place.  Because of Mercutio's death, Romeo feels emasculated.  


O sweet Juliet,
Thy beauty hath made me effeminate
And in my temper softened valor’s steel!



He feels that he has no other option than to bring terror down upon Tybalt.  


This day’s black fate on more days doth depend.
This but begins the woe others must end.



When Tybalt returns to the scene of the crime, Romeo announces his intention to kill him.  Gone is Romeo's aversion to fighting a new family member of his.  Romeo chooses to seek violent revenge.  He chooses to fight Tybalt, and Romeo chooses to kill him.  The result is that Romeo is banished from Verona.  


And for that offence
Immediately we do exile him hence.



Romeo's chosen actions are what cause him to be separated from his love and his wife.  Things go from bad to worse from this point in the play.  Juliet fakes her death, Romeo doesn't realize it's fake, and he chooses to kill himself.