Why does Macbeth seek the witches in Act III?

Macbeth wishes to seek out the witches, in Act III (scenes
i and iiii). Macbeth discusses the reasons as to why.

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Upon my head they placed a fruitless crow, And
put a barren sceptre in my gripe, Thence to be wrench'd with an unlineal hand, No son of
mine succeeding. (Act III,
i)

Here, Macbeth discusses
the part of the witches' prophecy that has come true.  Macbeth has become king. But, he
worries about what else he must do to keep his crown given that he will not have any
sons to pass the crown to.  Another part of the prophecy states that the crown will go
to someone outside of Macbeth's lineage.

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And betimes I will, to the weird sisters: More
they shall speak, for now I am bent to know, By the worst means, the worst. (Act III,
iii)

Here, Macbeth states his
intent to go back to the witches and ask them of his future.  He is concerned about what
he has done to this point to gain the crown; his concern, now, lies in what he must do
to keep the crown.