Your diagram has 6 cables and your question refers to 3 cables, so I'll just consider the simpler case.
- The distal end (effector) moves in 3D space (a volume) - task space. Whilst the accessible space is limited, it does require 3 axes to describe its terminal point.
- The input is 3 cables, so you only have 3 variables to play with to get a particular (accessible) distal end position. I suspect that is what you have called the configuration space. That looks like 3 dof to me.
- The mapping of 3 inputs to 3 outputs is possible, and that transformation has no superfluous components (meaning you have enough, but not too much, information).
- A little change to any input makes a little change to the output; there are no discontinuities in the transformation. In this sense, the manipulator can, theoretically, move infinitely small distances.
In a jointed manipulator (like an arm, with a hand), there can be lots of servos. However, at the end of the first finger, we still only need 3 numbers to describe its position in space, even if we chose to move 17 servos to get it there. Our 17-dof manipulator still only achieves the position in 3D, needing only 3 numbers to define its position.
(Note that we have ignored the orientation of the manipulated object. Yes, there's lots more to think about, yet.)