In Macbeth, what is the significance of the quote by Macbeth to Banquo in Act III in which Macbeth says, "We hear our bloody cousins are bestowed /...

Macbeth is telling Banquo that Malcolm and Donalbain are not admitting to Duncan’s murder.  The speech is foreshadowing Banquo’s death and Malcolm’s return.

When Macbeth killed Duncan so that he could become king, it was important that he also get the king’s two sons out of the way.  In order to do this, he framed Malcolm and Donalbain for the king’s death.  It was not hard for people to believe that one or both of them killed their father, because Duncan had just named Malcolm his heir.

It is important to Macbeth that Banquo not suspect him.  He wants to make sure that Banquo thinks that Malcolm and Donalbain are the real murderers.  In this scene, he is talking to Banquo just before he is supposed to be killed by Macbeth’s three murderers.  

Banquo is indeed suspicious of Macbeth.  He knows about the witches’ prophecies and he even heard one suggesting that his sons would be king.  Not soon after that, Duncan is killed.  Yes, anyone would be suspicious and worry that he would be the next target. 

Thou hast it now: king, Cawdor, Glamis, all,
As the weird women promised, and, I fear,
Thou play'dst most foully for't: yet it was said
It should not stand in thy posterity,
But that myself should be the root and father
Of many kings. (Act 3, Scene 1) 

Macbeth wants to make sure that Baquo has no suspicions in this speech.  He himself is worried that Banquo knows too much, and is also annoyed at the prophecy that Banquo’s sons would be king.  Before Banquo can get any ideas, he has to dispatch him.

To be thus is nothing;
But to be safely thus.--Our fears in Banquo
Stick deep; and in his royalty of nature
Reigns that which would be fear'd: 'tis much he dares;
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valour
To act in safety. (Act 3, Scene 1)

There is a nice parallelism to these two soliloquies.  Banquo is thinking to himself that he worries that Macbeth killed Duncan, and Macbeth is thinking to himself that he is worried that Banquo thinks that he killed Duncan.  Neither of these speeches is good news for Banquo.  Macbeth is suspicious of him, and Macbeth has the will to act, and fast.

Macbeth’s speech about Malcolm and Donlbain is supposed to allay Banquo’s suspicions long enough for him to be killed, but it also foreshadows, or hints at, Banquo’s death and Malcolm’s return.  By mentioning Malcolm to Banquo, Macbeth reminds us of his former murder.  The comment about Malcolm not confessing gives us insight into Malcolm’s true character as well and foreshadows his return.  Malcolm may have run away, but only long enough to form an army he can use to storm Macbeth's castle and get revenge on his father’s murderer so that he can take bath the throne that should be his.