Shakespeare makes the witches appear synonymous with wickedness and hell to compound how the inference with the supernatural is inherently evil.
During Hecate's want to manipulate Macbeth she refers to meeting at "the pit of Acheron".
Acheron was one of the rivers that ran through Hades (the underworld/hell in Greek mythology).
As they are depicted as meeting there, it reinforces how the witches are inextricablylinked with hell and thrive in this depraved environment.
Within Christianity, the foundations of religion in Christian society, there is the holytrinity, which is the idea that God exists in 3 forms: the Father, the Son, and the holyspirit.
As the witches always appear as a trio, they could be seen as the anti-trinity as they persistently temptfate and indulge in wicked actions.
Shakespeare intends to be condeming of the Witches and their wickedness as this would directly appease his patron, James I, due to him believing they were agents of the devil - this is seen in his book 'Daemonologie'.