II Shmuel 16:22
And they spread for Absalom a tent upon the roof; and Absalom came to
his father's concubines before the eyes of all Israel.
Note that the pasuk does not give the names or number of the women involved. Since they were the concubines of the king, Avshalom was declaring that he was now the rightful king. As such, he was claiming that it was his right to have relations with them.
This was done in order to ensure that Avshalom could never reconcile with his father. Had he reconciled with his father, the rebels would have been left for David to punish while Avshalom would have been let go. See the meforshim on II Shmuel 16:21. Thus, this ensured that those who supported his rebellion would be willing to come out into the open.
II Shmuel 20:3 now explains that they were treated as women who had become forbidden to their husband because of this. Thus, whether or not it had been considered rape, they were still treated as asur to King David.
However, since they had been the concubines of the king, they could not be allowed to marry anyone else. That pasuk states that there had been ten of them.
And David came to his house in Jerusalem, and the king took the ten
women [who were his] concubines, whom he had left to keep the house,
and he put them in a guard-house where he sustained them, but he came
not upon them. And they remained bound as widows [with husband yet]
alive until the day of their death.