Why 1-ethenyl-3-ethyl-5-ethynylbenzene has that name?
Ordering substituents by complexity has been abandoned long ago (before 1979) in favor of the much simpler alphabetic order. Here, ethenyl &lt; ethyl &lt; ethynyl.

Note, however, that simple substituents are ordered alphabetically before adding a multiplicative prefix, which means that "dimethyl" is ordered as "methyl".

Complex substituents are ordered by their first letter, e.g. isopropyl (I), dimethylphenyl (D)