Without going into demonology, in this case the text is explicitly clear that there was but one unclean spirit. Just one.
Even when the unclean spirit was expelled it was still just one and the text was careful to make a point about this. It is possible to have only one unclean spirit (and be a regular synagogue/church goer). It is more common that they operate in gangs certainly when the Bible mentions a number this is the case but this individual had one.
When the unclean spirit spoke through the man, and referred to us, it was referring to unclean spirits in general. This being option two. Jesus was a treat not just to that one unclean spirit but all unclean spirits.
Whether the unclean spirit invoked us to make himself feel better and not feel isolated and alone is debatable but it felt threatened by Jesus at a minimum. And whatever his fate was, he imagined had to be all the other demons’ fate too.
Unclean spirits know that they will all be cast into the lake of fire and destroyed. This unclean spirit wanted to know if this was going to happen presently because it wasn’t technically time yet (Revelation 20:15 would qualify daimons as those who were not written in the book of life but not going into details). It envisioned only two options, remaining in the man or destruction. It had no intention or reason to believe that expulsion was an option, because there was no history of demonic expulsion from a biological housing/body. The Jews however had some signs that would identify the Messiah and demon expulsion was one of the signs of the Messiah.
It’s not option 1 because the text speaks only of one and only one. It was the unclean spirit that invoked the plural
It’s not option 3 because demons have no allegiance to human beings, so it wasn’t worried about the humans being destroyed. It would be concerned that their (daimons) time is up.