What is the effect of what the Witches tell each other in the beginning of scene 3 in Act 1? And are these details important to the plot of the play?
This is a very thoughtful question. The witches do tell a few stories about what they are up to outside the events surrounding Macbeth here at the opening of Act I, scene iii. The main reason that they tell these stories is to advance the audience's understanding of them as characters, not to advance the plot of the play. It is a bit of a side-note to the action, but Shakespeare is giving key insight into why the Witches do what they do here in this conversation.
Shakespeare lived during a time when witches were really feared. The witch hunts that we know about from our own history were also ongoing in England, and people generally believed that witches were out stalking people to do them evil.
In this scene, Shakespeare is showing just this sort of evil-doing, spell casting, witch. Witch Two has been "killing swine," presumably to use in some incantation, brew or spell. Witch One has visited a woman who, when asked by the witch to give her some of her nuts, refused. Because of this, Witch One says that she will exact her revenge on the woman by plaguing her husband while he is at sea. She shows her charm, with which she will work her magic, the thumb of a sea pilot.
Shakespeare uses this scene to convince the audience of the dangerous vengeful nature of the Witches, but he is also a great business man of entertainment. Just as some people love to go to movies for the blood and gore, people went to the theatre expecting these sorts of gruesome moments. This scene is an opportunity to give his audience their money's worth, along with creating a useful picture of just what the Witches are up to. So, along with their conversation, he has one of the Witches pull out a human thumb to up the gross-out factor.
But no, in a word, this conversation is not meant to advance the plot of the play.