That is, those Aes Sedai who were captured but were not part of the Black Ajah. If the Three Oaths bind them to not use the One Power to harm another except for Darkfriends and shadowspawn, how could they be compelled to do so even under the influence of an A'dam?
Robert Jordan has answered a question similar to this. It can be found on Theoryland when searching for Third Oath
INTERVIEW: Jan 25th, 2005
TOR Questions of the Week Part II (Verbatim)
WEEK 21 QUESTION: Just how can an Aes Sedai be a damane? Aren't they bound by the Third Oath: to not use the One Power as a weapon except to defend their lives, their Warder's life, or another sister's life? Wouldn't they be useless as damane to the Seanchan?
Robert Jordan: The Aes Sedai captured by the Seanchan are indeed useless as weapons, except against Shadowspawn or Darkfriends, because they are bound by the Three Oaths, and that limits their value considerably since being weapons is a major use for damane. Damane are used for other tasks, however, including finding ores for mining (Egwene was tested for this, remember; it's a very valuable, and fairly rare, ability), for some mining operations where it would be too dangerous or uneconomical to use human miners (bringing ores out of the ground and refining them using the Power), and in some construction projects, especially where something very large or with a need for added strength is envisioned. The first two both require a high ability in Earth, which has faded considerably on "this" side of the Aryth Ocean and to a smaller degree of the other side, but construction projects and others things, such as producing Sky Lights, are well within the abilities of collared Aes Sedai. The Three Oaths don't inhibit them there at all.
In short, the Aes Sedai damane were not used as weapons, but instead they were used as tools to find ore, heal the injured, build buildings.
An excerpt from Knife of Dreams -
He had considered using another damane than Mylen. The tiny woman with the face he could never put an age to almost bounced in her saddle with eagerness to lay eyes on the High Lady again. She was not properly composed. Still, she could do nothing without Melitene, and she was useless as a weapon, a fact that had made her hang her head when he pointed it out to the der’sul’dam. She had needed consoling, her sul’dam petting her and telling her what beautiful Sky Lights she made, how wonderful her Healing was.
Karede's Point of View - Knife of Dreams (Chapter 36)
Even though Mylen (Sheraine Caminelle) wants to please the der'sul'dam very badly, she still cannot use the One Power as a weapon.
I finally found the quote referencing an Aes Sedai damane, Pura (originally known as Ryma Galfrey), who could not be forced to lie about the colour of a scarf:
Her agents reported that the women from the White Tower could not lie. It had not been possible to force Pura to tell even a simple lie, to say that a white scarf was black, yet that was not enough to be conclusive.
Seeds of Shadow - The Shadow Rising (Chapter 1)
This may work better as a comment on the word of god answer however IIRC the oaths only bind an Aes Sedai dependent on their perception IE an Aes Sedai could tell a lie if she believed what she said to be true
If the Seanchan managed to brainwash an Aes Sedai into believing that their enemies were darkfriends the power could be used as a weapon.