The Golden Trio of the Harry Potter series refers to titular character Harry Potter and his friends Hermione Granger and Ron Weasley. This fan-invented term tends to be used in a somewhat sarcastic way, referring to the fact that the trio can do no wrong in the eyes of authority figures who really ought to do more to discipline them and keep them out of trouble. On the other hand, they are the three main characters of the series, so it also refers to their heroism and triumphing-over-evil.
When found in badfic, The Golden Trio serves several different purposes, though it seems to be a rare find, seeing as so often one, both, or even all three of the original members of this so-called Trio are replaced (common occurances include Draco Malfoy, Neville Longbottom, and/or a Mary-Sue OC). Though these stories often use the Trio as a backdrop to the main "romantic" pairing, they can be seen as a deadly force on their own, known to create a Sue-generated SEP Field, have a dangerous amount of the Aura of Smooth, and seem to exude an aura of pure urple which is hard to surpass or even be near. Though they can be used appropriately in goodfic, it is an extraordinarily rare find. The reason for this rarity is that any of the three characters (and this could be all three at once) are usually bashed by the author, whether they are writing goodfic or badfic.