The fact the witches were mixing a "poisoned" concoction upon the entrance of MacBeth implies "trouble." Three witches circling around a cauldron, throwing in items such as "baboon's blood" foreshadows something dark and mysterious will happen. Hecate, the queen of the witches, "commends" the witches for their "pains," upon entrance to the witchery drenched stage. Hecate also uses a device similar to the royal we. She implies that the entirety of the populous will benefit from the outcome of the potion when she professes "everyone shall share i' th' gains.". Hecate also exits the stage with the song "Black Spirits".
When there is a knocking at the door, the second witch is aware of whom the visitor is. The riddling second witch states "something wicked this way comes," suggesting both MacBeth is an evil character, and the apparitions are going to make an entrance shortly. The witches might have been expecting MacBeth to arrive and were preparing for his entrance into the scene.
The three witches of Macbeth continually enter the stage with either thunder, or thunder and lightning. All three of the apparitions enter the stage with "Thunder. First" [or second, or third] "Apparition, An Armed Head" [or a Bloody Child or a Child Crowned with a tree in his hand], after MacBeth tells the witches to "call 'em" to the stage. After four sets of thunder, the audience realizes the apparitions are products of the witches' magic.
The apparitions profess three things MacBeth believes are...