I understand that in the first example, the molecule is chiral as there is no symmetry. For the second and third example, I see no symmetry in the 3d conformation after twisting the right hand side. In the third example, I do see symmetry if we twist the right hand side so that the carbon molecules are in one plane. What am I missing?
Edit: typo corrected. Also I was not clear in that the picture I attached is the answers the professor provided. I see why the first one is right as there is no symmetry. But I did not see why the second and third ones were considered chiral and achiral, as I saw symmetry in the third example and not in the second. From the answers though, I am understanding that we consider if the molecule is super imposable with it's mirror image?