I need 3 examples of how Shylock let revenge become his first priority and 3 examples of how this backfired in a negative way.

First, we'll discuss examples of how Shylock lets revenge become a first priority.
We can find two examples in Act 3 Scene 1, where Shylock is furious at both Jessica and Antonio. Shylock is apparently in a foul mood here; he laments that Jessica has betrayed him, and he's especially angry that she has run away with some prized jewels and a diamond he bought for two thousand ducats.
Tubal adds to Shylock's misery by divulging that Jessica spent "fourscore ducats" one night in Genoa and that she exchanged Shylock's prized turquoise ring for a monkey. The ring was a gift from Leah, Shylock's wife. Shylock is so angry at Jessica's actions that he wishes her dead at his feet with the jewels in her ear or buried in a coffin with the stolen ducats. Shylock is furious that he is spending inordinate amounts of money to find his thieving daughter. He laments that he has had "no satisfaction" and "no revenge." It is clear revenge is a first priority for Shylock here.
In the same interaction, Tubal also lets Shylock know that Antonio lost his ship while leaving Tripoli. Upon hearing this, Shylock rejoices. To understand how Shylock prioritizes revenge, you may be interested in his soliloquy in Act 3 Scene 1, where he lays out his rationale for revenge. Here's a short excerpt:

And if you wrong us, shall we not revenge? If we are like you in the rest, we will resemble you in that. If a Jew wrong a Christian, what is his humility? Revenge. If a Christian wrong a Jew, what should his sufferance be by Christian example? Why, revenge. The villainy you teach me I will execute—and it shall go hard but I will better the instruction (Act 3 Scene 1, Merchant of Venice).

Another example of Shylock prioritizing revenge can be seen in Act 3 Scene 3. Here, Antonio is in jail. He tries to plead with Shylock for mercy, but the latter rejects his suit. Shylock stubbornly maintains that he will have his bond and refuses to entertain lenience towards Antonio.

I’ll have my bond, and therefore speak no more.
I’ll not be made a soft and dull-eyed fool
To shake the head, relent and sigh, and yield
To Christian intercessors. Follow not.
I’ll have no speaking. I will have my bond. (Act 3 Scene 3)

Likewise, when he is summoned before the court in Act 4 Scene 1, Shylock reiterates his desire for revenge against the man he hates: Antonio. Shylock refuses to consider the duke's plea on behalf of Antonio, and he doesn't back down even after Bassanio offers him twice the amount of Antonio's loan (six thousand ducats).
 
Now, we'll discuss examples of how Shylock's thirst for revenge backfires against him.
First, when Portia offers Shylock three times the amount of Antonio's loan, Shylock rejects it. His insistence on his pound of flesh backfires on him. Portia proclaims that Shylock can have his pound of flesh, but he must not shed blood to do so. The penalty for shedding blood is the loss of all his "lands and goods" to the state of Venice.
Frightened, Shylock asks Portia to pay him her original offer of three times the amount of Antonio's loan.
 
However, Portia insists Shylock will have his pound of flesh and nothing more. Shylock refuses and tries to leave. For her part, Portia points out that a foreign citizen who tries to kill a Venetian citizen must give up half of his property to his victim and the other half to the state of Venice. So, three examples of how Shylock's priority for revenge backfires on him are:
 
1) Shylock ends up losing half of his wealth to the state of Venice and the other half to Antonio. The Duke offers to reduce Shylock's fine to the state. However, Shylock has to place the half he owes Antonio in a trust for Lorenzo (Jessica's fiance).
 
2) Shylock has to convert to Christiniaty.
 
3) Shylock has to will all his wealth to Jessica and Lorenzo upon his death.