My opinion:

COUNTESS DRACULA has Dracula in the title too; does that also belong in the Dracula series? 

No.  The title's a total cheat, there's zero connection to anything Draculaesque.

Does HORROR OF FRANKENSTEIN belong with the Peter Cushing Frankenstein series?

Yes.  There's very little continuity in the Cushing series, so why not include this one?  The various Universal series frequently changed actors (Karloff/Chaney/Lugosi/Strange, etc.), so the change in actors shouldn't disqualify this one from inclusion.  Of course, the fact that Cushing played the role in ALL the others does kind of muddy the waters, but I still include this in the series.  It's a Hammer film dealing with the doings of a Frankenstein and his Monster.  Totally fits.

BRIDES OF DRACULA is a different animal.  The character of Dracula is not in the film, but the character of Van Helsing does carry over from HORROR OF DRACULA and there is a reference to Dracula.  So there is a connection to the Dracula series, making that one a tougher call.