In act 4 scene III why does Malcom lie about himself to Macduff?
In Act IV, MacDuff has gone to England in order to get English military support and convince Malcolm to return to Scotland. He has suspicions that Macbeth killed Duncan and knows that his is a tyrannical ruler who must be stopped.
Remember that after his father's death, Malcolm fled to England so that whoever killed his father wouldn't be able to kill him. When MacDuff shows up saying that he wants to bring him back to Scotland in order to restore his role as king, Malcolm is not sure if he should believe him. Malcolm lies about himself in order to test MacDuff's loyalties and ensure that they lie with Malcolm. Before he will join MacDuff, he wants to make sure they are on the same side- a side that wants what is best for England.
It is myself I mean, in whom I know
All the particulars of vice so grafted
That, when they shall be open'd, black Macbeth
Will seem as pure as snow, and the poor state(60)
Esteem him as a lamb, being compared
With my confineless harms.
I grant him bloody,
Luxurious, avaricious, false, deceitful,
Sudden, malicious, smacking of every sin
That has a name. But there's no bottom, none,
In my voluptuousness Your wives, your daughters,(70)
Your matrons, and your maids could not fill up
The cistern of my lust, and my desire
All continent impediments would o'erbear
That did oppose my will. Better Macbeth
Than such an one to reign.(75)
Malcolm feels the need to test Macduff in order to make sure that he is not actually working for Macbeth. Apparently, Macbeth has sent others to England in an attempt to get Malcolm to return to Scotland so that Macbeth can make sure that he, as the rightful and legal heir to the throne, is killed. Malcolm tells Macduff,
By many of these trains hath sought to win me
Into his power, and modest wisdom plucks me
From overcredulous haste. (4.3.136-139)
So, Malcolm tells Macduff that he felt the need to lie in order to protect himself. If Malcolm tells Macduff that he is all manner of terrible things—lustful, avaricious, malicious—and Macduff still wants him to return to Scotland to rule, then that will seem like proof that Macduff hasn't come because he cares about Scotland but because he is serving the tyrant, Macbeth, like the others who have come before. On the other hand, if Macduff—upon hearing how awful Malcolm is—no longer wants him to return and rule, then that's a good indicator that he is honest and trying to serve his country. When Macduff tells Malcolm that he is not only unfit to govern but also "not [fit] to live," Malcolm knows that Macduff is telling the truth (4.3.121).