season 3 miller is full on ‘dear abby’ to alec’s personal & professional problems, and it’s glorious.
ellie helps him deal with daisy & claire’s tantrums both - a parallel - with claire, his roles as father and lover are purposely collapsed together (even though he wasn’t actually in a relationship with claire, their intimacy & creeping codependency is highly implied through much of the season; meanwhile, claire acts very childlike most of the time).
it’s notable that while hardy usually has no problem being brusque and demanding, unconcerned with people’s feelings, in these situations it’s been miller who has to convince him to take a tough hand. (both instances have to do with hardy’s feelings of guilt.) it’s one way of showing how their personalities have grown more similar over the show, but also a confirmation that miller's experience as a mother & in navigating human relationships makes her more effective in addressing claire’s petulance. the additional steel ellie has gained from alec has made her a stronger detective, but her interpersonal skills make him stronger as well. while hardy started season one as the more obvious figure of competence, broadchurch is always clear that he is less effective without a balancing figure.
lastly, as ellie confronts her son’s bad behavior, she has constantly questioned whether or not she has been a good mother - moments like these demonstrate that her disciplinarian side is harsh but fair.