Neither Risen nor The Witcher are similar to G3. Both lack in the open world aspect compared to the gigantic G3 while they have more depth instead. Risen is open and certainly at the upper end of the scale, but much smaller. The Witcher is not open, it has more or less big closed levels.
Risen is pretty much exactly Gothic 2 with a harmless interface.
The Witcher is a story-driven game with limited exploration. Setting and characters are similar to the Gothic series. I enjoyed it for the writing and the consequent neglection of teenie game standards. Choices & consequences are the core strength.
Risen / Gothic and The Witcher are brothers in spirit. All games are worth playing. The games are cheap, so I would say play them both.