There are several kinds of symmetry - translation, reflection and rotation are three.
And rotational has three axes and an infinite number of angles: 180, 120, 90, 72, 60, 45, 30, 15, 12 ... etc degrees.
UR suggests that certain symmetries are attainable - e.g. stack them vertically and you can rotate about that axis any number of degrees while leaving the configuration unchanged.
As UR points out, the OP seems to be talking about reflection or 180 degree rotational symmetry about an axis perpendicular to a 1-dimensional ordering of the six balls. Restricted to one dimension, reflection or 180-degree rotation symmetry can't be achieved.