After the Weird Sisters have delivered their predictions to Macbeth and Banquo, Ross informs Macbeth that he is the new Thane of Cawdor. However, when the sisters told Macbeth that this would happen, Macbeth didn't realize that it already had happened. Duncan gave him this new title in the scene prior, though Macbeth was not yet aware. The Weird Sisters didn't predict the future; they simply told Macbeth a fact that he didn't yet know. Therefore, when Ross delivers his news, it does seem to Macbeth and Banquo that the Weird Sisters correctly prophesied the future. Banquo says to Macbeth,
But ’tis strange.And oftentimes, to win us to our harm,The instruments of darkness tell us truths,Win us with honest trifles, to betray ’sIn deepest consequence. (1.3.124-128)
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence—(145)
Cousins, a word, I pray you.
Here are Banquo's lines from Act I, Scene III. The witches have just prophesied that Macbeth will be King, and Banquo's sons will be King - and Macbeth is clearly "rapt withal", absolutely amazed by the prophecy.
Banquo warns Macbeth that evil creatures ("the instruments of darkness") do not necessarily have to lie, but sometimes say true things ("tell us truths" and offer "honest trifles" - a trifle is a ) in order to tempt people into harming themselves (being won "to our harm") and making awful things happen ("betray's / in deepest consequence"). "Trifles" in this instance might mean "pleasant events", and "betray's" is just an elision of "betray us".
Banquo is warning Macbeth that - in line with the theme of the play and their own opening lines of this scene - what seems fair could actually be the work of something foul. In this play, people look like innocent flowers, when they are actually serpents - and Banquo wants his friend to look before he leaps.