Fate is definitely real in Macbeth, but it involves what is known as a self-fulfilling prophecy.  Macbeth receives information from the witches that one day he would become king, instead of waiting for fate or destiny to make him king, he takes matters into his own hands, kills the current king, damning his two sons with suspicion of having killed their father, and ends up getting crowned King of Scotland.

"The witches, through their ambiguous prophecies, represent a supernatural power that introduces evil into Macbeth. Their equivocations—the intentional stating of half-truths—conceal the sinister nature of their predictions, and Macbeth does not consider the possibility that they are trying to deceive him."

The witches are agents of evil, making mischief at Macbeth's expense, toying with his human frailties, watching him drown in his own ambition, wallow in over confidence as he destroys the very fabric of nature in Scotland. 

"The Weird Sisters themselves do not have the power to enact a diabolic course of events such as that which ensues in Macbeth; rather, their power lies in tempting humans like Macbeth to sin."

In fact when Hecate, the queen of the witches discovers what they have done, she takes charge of the second set of prophecies because they have not exploited the human Macbeth enough.  She wants him to twist in the wind unaware that he is setting in motion his own destruction and that he is powerless to stop it. She wants to instill in him a sense of over-confidence, something she sees as a great flaw in humanity that should be preyed upon to unleash more chaos, mayhem and death.

The witches are agents of evil, they engage in prophecy not to help Macbeth, but to stir his deepest, darkest emotions, with the belief that as a human, his greed and lust for power will destroy many lives including his own.  

In William Shakespeare's Macbeth, are the witches only agents of evil or can they be called the fates?

The witches in William Shakespeare's Macbeth have been described as numerous things. They were called "instruments of darkness" and "fantastical" by Banquo and  "imperfect speakers" and "black hags" by Macbeth. Some readers believe that the witches are truly evil (based upon stereotypes of witches); and others may believe them to be the voices of the Fates (as seen in Greek mythology--ones who would determine a man's destiny). That said, the witches only provided Macbeth with a glimpse into his future; it was his hand which forced the removal of Duncan from the throne. 

Therefore, depending upon one's individual interpretation, a reader could support his or her own defining of the witches using different textual support. If following Banquo's point of view, one could deem the witches as evil. If following Macbeth's train of thought, one could deem the witches as evil as well. If looking at the allusion made ot mythology, one could easily state that the witches represent, or symbolize, the Fates. Lastly, one could suggest that the witches symbolize opportunity and the extent one may go to if another believes the opportunity exists.