Why does Macbeth in act three keep his attack on Banquo a secret from Lady Macbeth?

Secret? No, Macbeth does not keep the imminent murder of Banquo a secret. He does more than suggest what he plans to do in relation to Banquo and Fleance; he just leaves the specifics as a little surprise for his wife. It's as if Macbeth is using murder as a present for his wife which she will thank him for after she opens the package. Actually, Lady Macbeth, in this little exchange, suggests the murder first (Act 3, scene 2):

MACBETH:

O, full of scorpions is my mind, dear wife!

Thou know'st that Banquo and his Fleance lives.

LADY MACBETH:

But in them nature's copy's not eterne.

MACBETH:

There's comfort yet; they are assailable.

Then be thou jocund. Ere the bat hath flown

His cloister'd flight; ere to black Hecate's summons

The shard-borne beetle with his drowsy hums

Hath rung night's yawning peal, there shall be done

A deed of dreadful note.

LADY MACBETH:

What's to be done?

MACBETH:

Be innocent of the knowledge, dearest chuck,

Till thou applaud the deed.

You see, he just wants her to wait a bit, innocent only in the details. The two of them make a sick, murdering couple, don't they? The irony is that, at the big dinner they are throwing this night, she will learn only to well what has happened to Banquo and his son Fleance.