Macbeth sent me, he didn't know if you were competent enough to do the job.
Let it come down
It will be rain tonight.
We have lost the best half of our affair.
The two murders are unexpectedly joined by a third man, who was sent by Macbeth, creating an ominous mood considering much of his identity is unknown.
Banquo is looking up at the sky as he talks to his son, which further foreshadows his death as he is looking in the direction of the heavens.
As Banquo was attack and killed by the three murders, Fleance was able to escape proving that fate cannot be avoided. Fleance survived the attepted murder and can now become king, just as the the witches prophesied.