cable 1 has to become too stiff because you are restricting the motion of the cable 1 (along X axis) by connecting cable 3 to the ground. So you are adding stiffness to cable 1 now because now it has a support somwhere along its length (to be exact, at the location where you are connecting cable 1 to cable 3). So I don't think there is any problem with the results.
You might not be seeing what you expected, but that perhaps is because you didn't model it correctly which would give you the desired results.