In the second half of 3.1, Shylock has a conversation with Tubal, another Jew. What is Tubal's relationship with Shylock like? What can we glean...

Tubal apparently works for Shylock.  Shylock sent him to Genoa to find Jessica, Shylock's daughter, who had run away and taken a lot of her father's money with her.

Tubal provides a strong contrast to Shylock in this scene.  Shylock is already beside himself when the scene begins.  Tubal is calm, blunt and matter-of-fact.   He has bad news to give Shylock, and he appears to know that Shylock will react with a tantrum.  Throughout their conversation, he gives Shylock a succinct answer, lets him rant and rave himself out, and then gives another calm, one-sentence answer.  He does not try to soften the bad news, apparently realizing that this is futile.

For example, after Tubal gives the first sentence of his report ("I often came where I did hear of her, but cannot find her"), Shylock flies into a long rant about how useless it was to spend money trying to find Jessica, how he wishes she were dead, etc.  

Tubal waits this out.  

Then he flatly contradicts Shylock's claim that there are "no tears but o' my shedding":

"Yes, other men have ill luck too. Antonio, as I heard in Genoa -"

Shylock interrupts frantically: "What, what, what? Ill luck?  Ill luck?"

Tubal, unruffled by this interruption, calmly finishes his sentence: "... Hath an argosy cast away coming from Tripolis." 

Then, in between Shylock's exclamations, he continues to give Shylock one-sentence bits of news, both good and bad, seemingly indifferent to the effect it will have on Shylock to hear that, for example, his daughter spent a fortune one night in Genoa, or that "Antonio is certainly undone." 

From this I gather that Tubal knows Shylock fairly well and has probably done errands for him before. He is aware that Shylock turns nasty when things don't go his way.  He does not really care how things turn out for Shylock, and he has become numb to (or hardened himself to) Shylock's tantrums.  Tubal apparently does not think it's his business to calm Shylock down, only to report on what he was asked to find out. 

It's possible that Tubal is not very bright.  We do not see any more of him in the play, so it's hard to tell.  But considering that he was able to get himself to Genoa and find word of both Jessica and Antonio, probably he is not stupid, just apathetic around Shylock because of Shylock's typical behavior.