What does Banquo mean in Macbeth, Act 3, Scene 1, when he says "Thou hast it now: King, Cawdor, Glamis, all, as the Weird Women promised; and I fear, Thou played'st most foully for't..."?

This quote also shows why Macbeth fears Banquo and wants him dead. Banquo is intelligent, suspicious, and dangerous. He is sure that Macbeth murdered King Duncan, and Macbeth is well aware that Banquo is sure of his guilt. Macbeth, however, does not know what Banquo might do about it. Banquo has all the qualities Macbeth ascribes to him in one of his own soliloquies. Banquo is the one man whose being Macbeth fears. He is probably afraid that Banquo could organize a plot against him because the man possesses leadership qualities among other things. Macbeth is like Claudius. Macbeth is wondering what Banquo is thinking and planning, while Claudius is wondering the same things about Hamlet. Banquo is wise enough to hide his thoughts and feelings from everyone, including Macbeth. He is treating Macbeth with the utmost courtesy and respect. It is obvious to both men that Banquo would be closer to realizing the witches' prophecies with Macbeth out of the way. Banquo's offspring would be closer in line of succession to the throne. 

What does Banquo mean in Macbeth, Act 3, Scene 1, when he says "Thou hast it now: King, Cawdor, Glamis, all, as the Weird Women promised; and I fear, Thou played'st most foully for't..."?

This famous quote opens Act III, and it is important to realise how vital this quote is. Banquo has silently witnessed what has transpired in Act II and how Duncan has been assassinated and Macbeth has seized power. He alone was there with Macbeth when the witches predicted that Macbeth would become Thane of Cawdor, Glamis, and then King, and now we see that he suspects Macbeth of having gained these prophesied titled through foul means, as Banquo says tha the fears that Macbeth "play'dst most foully" for the titles he has won. This is a very important soliloquy in the play therefore, as in it Banquo voices the suspicions that he has of Macbeth and internally debates the truth of the prophecies of the "Weird Women." For Banquo, too, received prophecies, and surely if the prophecies that applied to Macbeth came true, then those relevant to Banquo will come true as well. However, as that involved one of Banquo's heirs becoming King, this is potentially a treacherous thought, so he is quick to be silent when he hears other characters enter.

What words in this Macbeth quote link to themes of betrayal or ambition?

"Thou hast it now: king, Cawdor, Glamis, all, / As the weird women promised, and, I fear, / Thou play'dst most foully for't..."

The first sentence of Banquo's quote in act 3, scene 1, relates to the witches' prophecies and Macbeth's desires, while the second sentence reveals his growing suspicions regarding Macbeth's involvement in Duncan's murder. Specifically, the words "thou has it now" and "played’st most foully" are closely associated with the concepts of betrayal and ambition. The words "thou has it now" refer to Macbeth's prestigious titles, particularly that of king of Scotland, which was the position he desired that stimulated his ambition.

Banquo is also saying that he believes Macbeth engaged in foul play to attain his position as king. The foul play refers to Macbeth's betrayal of King Duncan. The audience also recognizes that Macbeth's primary motivating factor for committing regicide is his ambition, which was aroused when the first prophecy regarding his new title Thane of Cawdor was confirmed by Duncan's messengers.

What words in this Macbeth quote link to themes of betrayal or ambition?

"Thou hast it now: king, Cawdor, Glamis, all, / As the weird women promised, and, I fear, / Thou play'dst most foully for't..."

In tragedies, the heroes, who are people of status and admirable character, are often brought down by something Aristotle called a hamartia, a mistake which ultimately causes their downfall and destroys their character. For example, in the play Othello, the brave hero's character flaw is a jealous nature, and it drives him to kill his wife, which ultimately brings about his being dishonored and dying by his own hand. In Macbeth, Macbeth's character flaw is greed, and it drives him to murder King Duncan. In the quote your question is referencing, Banquo is reflecting upon how Macbeth's greed has likely led to him to murder the king.

He says, "Thou hast it now: king, Cawdor, Glamis, all." Banquo was present with Macbeth when the three weird sisters, or witches, gave Macbeth the prophecy that he would be Thane of Cawdor and king. Banquo not only saw the strange witches, he also saw the effect their words had on Macbeth. The first prophecy, that Macbeth would be Thane of Cawdor, came about soon after the witches disappeared. The fulfillment of that prophecy grew like a seed of poison in Macbeth; after he had a taste of favorable destiny coming true, his mind opened to the potential that he could be king. He begins to covet that position of power and ultimately takes fate into his own hands. He tries to help fate along, and he ends up losing his good name, his integrity, and all his titles because he cheats. That is the paradox of fate in many Shakespearean tragedies: "Fair is foul, and foul is fair." The fair prize of being king is so desirable that it leads to foul play and a foul ending for Macbeth.

What words in this Macbeth quote link to themes of betrayal or ambition?

"Thou hast it now: king, Cawdor, Glamis, all, / As the weird women promised, and, I fear, / Thou play'dst most foully for't..."

In terms of ambition leading to betrayal, the last line is the most relevant part. Saying that Macbeth played foully for all the titles he has at this point in the story is emphasizing that he has been competing (playing) but with some sort of unfair (foul) advantage. The last line also emphasizes that Macbeth is so focused on winning this "game" that he will betray anyone or anything that would try to impede him from achieving his ambitions. You could definitely look at the words "I fear," also, since those are relevant to the idea of betrayal that has already occurred as well as betrayal that is yet to come in the narrative.