Complex sentence: Two of them make a quasi covalent bond and one remains unpaired; the overall symmetry is however axial (trigonal C 3V) that can be imagined as those three unpaired vacancy electrons continuously exchange their roles.
Split into simpler sentences: Two of them make a quasi covalent bond and one remains unpaired. The overall symmetry, however, is axial (trigonal C 3V); one can visualize this by imagining the three unpaired vacancy electrons continuously exchanging their roles.