No, SO3 does not contain a C3 axis but no horizontal plane. SO3, also known as sulfur trioxide, is a trigonal planar molecule. It does have a C3 axis of symmetry, which means that it can be rotated by 120 degrees around its central axis and will appear the same. Additionally, it also possesses a horizontal plane of symmetry, which passes through the molecule's center and bisects the S-O-S angle.
