Quote 1
ANTONIO
Hie thee, gentle Jew.
The Hebrew will turn Christian; he grows kind. (1.3.191-192)
Antonio's sarcasm is pretty blatant here—he cannot fathom the possibility that Shylock the Jew is just being "kind." We also notice Antonio's use of the word "gentle," a term that shows up quite a bit in this play. For Antonio and many of the other characters, "gentle" means a few things: 1) considerate behavior, 2) aristocratic heritage, and 3) gentile (Christian). In other words, Antonio is saying that Shylock will never be "gentle" (considerate or upper class) because he's not a Christian.
Quote 2
ANTONIO
Thou know'st that all my fortunes are at sea;
Neither have I money nor commodity
To raise a present sum. Therefore go forth:
Try what my credit can in Venice do;
That shall be racked, even to the uttermost
To furnish thee to Belmont to fair Portia.
Go, presently inquire, and so will I,
Where money is, and I no question make
To have it of my trust, or for my sake. (1.1.184-192)
Wow, the wealthy Antonio sure does love his BFF. Here he says he'd loan Bassanio the money he needs to woo Portia in style, but he can't because all his dough is tied up "at sea." As an alternative, Antonio says Bassanio can use his (Antonio's) good credit in order to secure a loan. (Basically, Antonio's going to be a kind of co-signer.) This is a really generous and risky offer for Antonio to make because, as we know, Bassanio is terrible at managing his money, which is why he's always sponging off his friend.
Quote 3
ANTONIO
Well, tell me now what lady is the same
To whom you swore a secret pilgrimage,
That you today promised to tell me of? (1.1.126-128)
Bassanio's love life is the first thing Antonio brings up with Bassanio when they're alone together in the play. Maybe he's just one of those guys who likes to gossip, or maybe Bassanio has been on his mind. Bassanio's new courtship seems likely to be the source of Antonio's sadness, as it's at the forefront of his mind. Antonio clearly cares deeply about Bassanio as a friend, and he's facing the fact that he might lose him to this woman.