There should only ever be one companion and AT MOST a sub-companion; either temporary third members like Jack/River, or Rory sort of worked as... the companion's companion...
But it's not an ensemble show and doesn't work as an ensemble show. The main focus needs to be The Doctor with one "regular" companion to bounce ideas off and keep them grounded. They understood that way back in the Tennant era with (I think it was) Donna pointing out that The Doctor shouldn't be on his own because he loses perspective. That's why s/he needs a companion... as a mirror I suppose... but it only really works when it's one person.