When defining a list view, the fields which are available are specifically defined as one of three states:

• Always on

• Default on, but hideable

• Default off, but showable

Any fields that aren't defined in the view at all, are not loaded from the database and can't be shown. For any view that has no fields that are showable/hideable, the triple dot won't show.

To make fields showable/hideable, in the list view definition you need to add optional="show" or option="hide" (depending on whether you want to default shown or hidden for new users). This can be done through Odoo studio or manually by extending views.