Witch Creek Road S4 E9 Commentary
SUMMARY: Nadine, focused on the sacrifice of Oli, doesn't notice Faye approaching from behind. Faye easily captures her, intending to make her a sacrifice to her shark god. The get attacked by one of the Deep Ones, which Faye easily kills. The cultists arrive, who are enraged when they see that Faye has killed a Deep One. Faye and the cultists begin to fight, with Nadine caught in the middle.
"AT THIS POINT, THEY DON'T EVEN NEED YOU": Faye points out that the cultists don't even need Nadine at this point. This all comes back to the power of three. For example, there are three Wolves. In Christianity, God has three aspects (the Father, the Son and the Holy Ghost). There are three Furies, three Morrigan, three Norns, etc.
In this case, the cultists worship three gods: Father Dagon, Mother Hydra, and Cthulhu. They require three people for their sacrifice. Even if the Deep Ones outright kill Oli, they still have Dave, Ashlee and Grant.