Triss (Chapter III)
To sleep with Triss in Chapter III, you'll need to do three things:
  1. Complete the quest The Source and send Alvin to Triss.
  2. Teach Alvin discipline (set rules for him, support Triss' decisions).
  3. Give Triss any ring with rubies in it. If you don't have one, you can buy a Silver Ruby Ring from a merchant in the Trade Quarter's marketplace.
"The little one's sleeping, so I can show you real magic...."