Why is Shylock not willing to give 3000 ducats?

From what I can see, you are asking why Shylock isn't willing to lend Bassanio three thousand ducats.
According to the text, there is a reason for Shylock's initial reluctance to part with his money. We can find our answer in act 1, scene 3.
For his part, Bassanio assures Shylock that Antonio will guarantee his loan. However, Shylock personally despises Antonio, on account of the fact that the latter is a Christian and lends money without interest. Shylock believes that Antonio's practice of lending out money "gratis" lowers interest rates in Venice and cuts into his profits. So, Shylock doesn't like the fact that Antonio will be guaranteeing Bassanio's loan.
Additionally, Shylock remembers Antonio's past treatment of him and is still angry about it.

You, that did void your rheum upon my beard
And foot me as you spurn a stranger cur
Over your threshold! Moneys is your suit.
What should I say to you? Should I not say,
“Hath a dog money? Is it possible
A cur can lend three thousand ducats?”



“Fair sir, you spet on me on Wednesday last;
You spurned me such a day; another time
You called me ’dog'—and for these courtesies
I’ll lend you thus much moneys?”


(act 1, scene 3)


So, we can see why Shylock is initially reluctant to lend Bassanio the three thousand ducats: He knows that his despised enemy, Antonio, is the one guaranteeing Bassanio's loan. For his part, Shylock holds a grudge against Antonio and is upset that he must conduct business with his sworn enemy.