Let us remember that imagery is any passage or section that paints a strong image with the words that it uses so that we are able to picture the scene in our head. Of course, a number of different techniques can be used to create good imagery, but have a look at this example from Act I scene 2, when the Sergeant describes to King Duncan what has been happening in the battle that he has just come from and the state of the two armies and their conflict:
The Sergeant chooses to use this excellent example of imagery to describe what is going on with the conflict. He compares both armies to two exahusted swimmers that are trying to wrestle with each other and choke the other, but are finding it difficult because they are so absolutely exhausted and have no energy left. Such imagery helps us to really visualise the action being described and to picture it in our mind's eye.
The word "diction" simply means word choice. Shakespeare uses diction to create character. Duncan and Malcolm, for example, use educated speech; the three sisters, the simple repetitive language of spells or folk rhyme; and the Sergeant, more colloquial language appropriate to his background.
The Sergeant's speech is an example of ekphrasis, or vivid description, using imagery to convey the quality of the battle and of Macbeth's fighting to the audience, as in the lines describing the violence of Macbeth's attack:
with his brandish'd steel,
Which smoked with bloody execution
The most obvious use of symbolism is the use of numbers by the witches, which have significance as part of the numerological beliefs (deriving from Egyptian and Pythagorean traditions) underlying witchcraft and alchemy. Although in Christian context the number three most often symbolizes the Trinity, in this context it makes us think of darker chthonic triads such as the Fates and Furies (both being represented normally as three old women) and the moon goddess (who has three distinct aspects) often associated in the Renaissance with witchcraft.