After killing King Duncan and becoming king himself, Macbeth is paranoid and fears any threat to his title. Earlier in the play, the witches prophesize that Banquo's descendants will inherit the throne. To prevent the witches' prediction from being realized and in an effort to secure his position as king, Macbeth decides that Banquo and Fleance must be killed. Although Macbeth and his wife previously conspired to kill Duncan and take his throne, Macbeth elects not to tell his wife of his plans to have Banquo and Fleance killed.
There are several possible reasons why Macbeth chooses not to inform his wife of his plans. He vaguely tells her that something terrible will happen and when she presses him to elaborate, he replies, "Be innocent of the knowledge, dearest chuck, / Till thou applaud the deed." It is possible Macbeth is trying to protect his wife. He tells her to "be innocent," which can be interpreted as him wanting her to have plausible deniability in the event that the murders are investigated. If she is not privy to his plans, her ignorance and innocence will be more believable if she is ever suspected of being involved in the killings. Though it is his wife's idea to kill Duncan, Macbeth appears to be trying to shield her from repercussions of their crime and avoid implicating her in future murders.
Another possible explanation for Macbeth's exclusion of his wife in his decision is fear that she will not approve and will try to dissuade him. He tells her to remain ignorant until she can "applaud the deed." Perhaps he realizes that she will not approve of his plans, and does not want to include her until he knows she cannot oppose or interfere. He does not refuse to ever tell her of his plans, but wants to wait until after the deed is committed, so there will be nothing she can do about it.
A less likely, but still possible, reason for Macbeth's omission is forgetfulness. He has much on his mind by the time he decides to have Banquo and Fleance killed. Perhaps not telling his wife of his plans is an oversight. Maybe he does not feel that it is worth mentioning or troubling her with.
Macbeth seems almost protective over Lady Macbeth here when he tells her to be "innocent of the knowledge until [she] applauds the deed". Yet, judging from the tone of their conversations just prior to this, it may be simply that he knows that they are not in agreement and doesn't want to argue with her about it. Lady Macbeth has just come to the understanding that they've sacrificed everything to get what they want, but they're not happy ("Naught's had, all's spent,/Where our desire is got without content.") She isn't interested in further violence--the cost to their peace of mind isn't worth it: "'Tis safer to be that which we destroy,/Than, by destruction, dwell in doubtful joy". Macbeth hasn't actually avoided telling her--in fact, he tells her outright that he thinks they have "scorched the snake, not killed it" and he also tells her of his mental agitation. She doesn't want to hear it. She tells him to stop thinking about it (what's done is done) and tells him he must leave it. She asks him what is to be done about the Banquo problem, but within the context, it seems more of a rhetorical question, a comment, than a request for information.
It may be that he is taking on the man's role, but he does not sound manly--in face he seems more like a coward than a brave man. He speaks as if he expects her to be proud of his actions (she will applaud the deed) but he hasn't been listening.
Of course, it could also be something as simple as omission. Macbeth is pretty caught up in his dreams and aspirations at this point (in addition to trying to cover his tracks and seem normal). It's very possible that he just didn't think to tell Lady Macbeth...may have simply just slipped his mind. While it seems like a pretty large event to "forget" to tell your wife, you have to remember how much is happening around Macbeth right now and how quickly it's happening. Not telling her may not have been intentional at all - although he most likely wouldn't have told her even if it came to mind in order to protect her as best he could.
It is Macbeth's fears and guilt that motivate his plan to kill Banquo and Fleance at this point in Act III, Scene ii. He wants his own sons to succeed him as king, and he's afraid that won't happen if he doesn't have Banquo and his son killed. The Macbeths have no children at this point, but he's obviously planning for Lady Macbeth to give him sons. One reason he doesn't want to involve her in his plans of murder may be that he wants to keep the mother of his sons-to-be out of any more killing. He allowed Lady Macbeth to bolster his courage and make him manly enough to kill Duncan, but he doesn't need that encouragement now. He knows what must be done, and he has nothing to lose by killing two more. This is another reason why he feels no need to tell Lady Macbeth of his plans. It's unnecessary to involve her in any more murders now that the first has been committed.
At the banquet, Macbeth sees the ghost, showing us how conflicted he still is. The ghost that only Macbeth can see is his guilt overwhelming him, causing him to publicly express his fears and guilt for the first time. Lady Macbeth tells him to hide his fears, but she has to send all of the guests away.
I don't think Macbeth is blinded by power and ambition at this point. He's still motivated by his fears and guilt. It's not until he decides to kill Macduff and his family that the power and ambition consume him in Act IV.