I wrote this up once for a different site, but I don't seem to have kept it. So I'm starting from scratch.
I like the Trio. I think they're nifty. I think they're especially deeper than most people give them credit for in the context of S6. Throughout the season, the Trio can be seen as a symbolic external manifestation of Buffy's depression.
"Huh what the whosit?"
Yeah, I know. Bear with me. Let's take a look at the episode, Life Serial, as that's the episode that establishes this metaphor.
In Life Serial, the Trio "tests" Buffy, each one using their own specialty. And each of these tests portrays a different aspect of depression in a metaphorical sense.
The first test belongs to Warren, who makes a little time-skipping device. Let's not try to figure out the logistics of it cause it'll make your head explode. Here's the relevant part: Time works funny when you're depressed. You space out. Time moves faster or slower at times. It's disorienting. And you often feel as if the world is moving around you while you're stuck in one place.
Which is exactly what this experiment did to Buffy. It took this real-life symptom of depression and turned it into an over-the-top metaphor. Buffy sees time speed up in short bursts. Suddenly, the fast forward button seems to "stick", and everything speeds by while Buffy has to cower under a table for protection. The world is literally moving around her.
Fortunately, this is due to evil lint in the show, and the Trio self-destruct the technology.
The next test is carried out by Andrew. He summons demons to attack Buffy while she's working construction with Xander.
Again, we have a connection to depression. True fact: work is hard when you're depressed. The depression is an obstacle you need to overcome in order to work efficiently. And this obstacle is literally manifest in the demons that the Trio sends after Buffy.
The consequences of this are very true-to-life with depression. Buffy's seen as being "crazy", unstable, and a poor worker. There's no evidence of demons in the attack, mirroring the fact that depression is often an invisible illness. And nobody believes her. So many people never tell their employers about their mental illness because it's seen as a lame excuse for slacking off or doing a poor job. Just like the demons were seen by Tony in this instance.
And, in the end, Buffy gets fired.
Finally, we have Jonathan's test, which also involves time. This is a time loop, though, which causes Buffy to be stuck repeating the same scene while working retail for Giles and Anya.
Real life connection? You betcha. Monotony. Repetitive monotony. That's what life tends to become when depressed. Everything's the same. And there's no way to break out of it. You're stuck in a loop.
Which is basically what happens to Buffy in this instance. It takes several attempts for her to get herself out of the loop.
So throughout the day, Buffy's been tested by the Trio with trials that have striking parallels to the symptoms of depression. After all this, Buffy does what a lot of depressed peeps tend to do: she turns to drinking. She retreats to Spike and ends up having a bit of a bender.
This is significant.
Warren: Guys. Think about this. We took on the Slayer. We’ve got all kinds of stuff in the computer now. Speed, strength, reaction time. We’re getting what we need to really be a threat to her. We tested her, faced her, and we survived.
With the Trio established as a physical symbol of Buffy's depression, they recognize in this episode that they can be a threat to her. They discover that Buffy is so affected by the trauma and turmoil of her life that her duty as a Slayer has been compromised.
Now let me zoom out from that one episode to focus on the season, as a whole. The Trio were introduced in Flooded, which is the first "back to life" episode for Buffy (Bargaining and After Life both deal with post-death issues). The depression is with Buffy from the very beginning of her attempt at living again.
And after she's tested by it in this episode and found to be vulnerable, the Trio lays low for a few episodes. They don't appear again until Smashed, but when they do, they're doing crime without any concern for the Slayer.
Not only that, but the Trio are the ones who confirm for Spike that his chip is working. Once Spike discovers that, he's free to tell Buffy she came back "wrong", which only deepens her depression.
Their role in Gone is obvious. Much like Life Serial, they're the ones that inflict the invisibility on Buffy, which subsequently deals with a boatload of depression issues.
The really interesting episode, though, is Dead Things. Up until this point, the Trio had been a nuisance. They'd been an annoyance, at most. Almost nobody could take them seriously.
Rather like Buffy's depression which manifest as low-level misery with Buffy barely even questioning what was wrong with her.
However, it's in DT that the Trio show themselves to be actual villains with their attempted rape and actual murder of Katrina (And, honestly, that's a scene I could and should do an entire entry on. It's a brilliant scene. One of the best of the series). With this, Buffy's depression also finally shows the depth of its seriousness with her beating of Spike and her final plea to Tara that she be "wrong".
It's in this episode that Buffy's depression and the Trio collide in such a deliciously metaphorical and literal way. Again, the Trio act as an external representative of the state of Buffy's depression. Just as Buffy goes through a crisis in this episode, so do the Trio as they show their true colors. It's an episode the reveals the depths of the Trio and Buffy's depression and makes the final link between the two.
The Trio is intricately connected to Buffy's depression. As her depression deepens, the Trio gets worse.
However, DT is the height (or lowest point) of Buffy's depression. As You Were is a significant episode in her recovery, but we don't see the Trio again until Normal Again. We start to see the flipside of the pattern: as Buffy's depression improves, the Trio begin to fall apart. They are the symbol of her depression. As Buffy gets better, they start to disintegrate.
In Normal Again, Buffy discovers the Trio's hideout. The Trio reacts quickly and sends a demon after her. And, unsurprisingly, the demon spurs on another depressive episode. When threatened, the Trio lashes out to attempt to worsen Buffy's depression.
This time, she hallucinates, sending herself to an actual mental hospital.
This incident is just an episode, though. Buffy is able to snap herself out of it, proving that she's stronger than she had been at the beginning of the season. As she does, the Trio deal with more and more internal discord.
By Entropy, the Trio have stopped focusing on the Slayer. They're looking to get some money and split up. Buffy is pretty much recovered. She's got her purpose in life back. She's Slaying again. She's holding strong against the sexy temptation that is Spike. She doesn't want to escape anymore. She's trying to live. And so there's no need for the Trio anymore. They're looking to go their separate ways once they get some money.
Of course, also in Entropy, the gang logs onto the Trio's video feed and spies on Spike and Anya having their solace. This causes massive chaos among the Scoobies.
Things get serious around this time. Instead of playing victim to the Trio (her depression), Buffy actively seeks to take care of them. In Seeing Red, Buffy confronts the Trio head-on and utterly defeats them. This is where the Trio is finally split up, and Warren does a bit of last-ditch vengeance (killing Tara in the process).
It's no coincidence that Warren kills Tara in the same episode that Spike almost rapes Buffy. As Warren, her depression, acts out in an attempt to hurt her, Buffy also receives a very real consequence of her earlier behavior while depressed. The two are connected.
By the end of the season, it's only with Warren dead and the other two members of the Trio running to Mexico that Buffy can fully recover by crawling back out of her grave again.
Throughout the season, the Trio were used as a sort of barometer for Buffy's depression. At times, they were the direct causes of the manifestations of her depression (Most noticeably in Life Serial and Gone). When Buffy hit rock bottom, the Trio became their most villainous (Dead Things). And when Buffy began to recover, the Trio began to fall apart and act out erratically.
And I'm never good at ending these things. I hope this all makes some modicum of sense. Please let me know if anything's unclear. I would love to hear other people's thoughts. :)