Shinji Matou, Gilgamesh, and Cu Chulainn appeared at the same time because they are now in the same camp.
Cu Chulainn's Master is Kirei Kotomine. Although Kirei Kotomine has not yet shown himself, he is an unignorable party in this Holy Grail War as both a Master and the overseer of this Holy Grail War.
Moreover, because Kirei Kotomine participated in the last Holy Grail War, his Heroic Spirit at that time was Gilgamesh. In the last battle, due to the Blood Gate Adventure Group's need to implement the Artificial Holy Grail plan, the Holy Grail had to be contaminated. Only by achieving this condition could they create the Artificial Holy Grail.
Because of this, Stan once led people to clean up the Fourth Holy Grail War. They slaughtered all the Lone Contractors, allowing the plot to advance to the final battle, which in turn would lead to the successful contamination of the Holy Grail.
