This, of course, defines their functions - Willow thinks, Buffy does and Xander holds them all together. And it works very well, but that's to be expected.
What's funny is that it seems to apply to other trios - not all other trios, but many of them.
One example that just jumps to mind is Hermione/Ron/Harry. It means that Ron is necessary, that he's the glue. Another is, as thatcrazycajun says in his wonderful song "Simple Country Doctor", Spock/McCoy/Kirk.
But, as a DC fan, I was thinking about the Big Three of that universe. And that's when it got interesting, at least to me.
Because. Batman is clearly the Head. There's no real question about that. Neither Clark nor Diana are stupid, by any means, but Batman is the brains.
But. In these trios - all of these trios - the Hands are the Hero. The Slayer, the Captain, the Chosen One. The one that gets the capital letters. And in the DCU, the Hero is Superman. Not in the same sense as the others because he's part of a huge shared universe with multiple stories and he's only the star of his own, but he's the very symbol of Hero - he's the man with all the powers who uses them only for good, and who really shy and retiring ("mild-mannered").
So he should be Hands. And he's NOT. He's Heart. Wonder Woman is Hands. Wonder Woman is the warrior of the three of them. She will do all she can to avoid killing or violence, and she's the soul of compassion, but in the end, she will do what she has to do. It's why she blinded herself with Medusa's venom and why she killed Max Lord (although I maintain it was at his request.) And if she has to do it, she will, and with no regrets.
Clark? He does what he does because he cares - not the nobility of the princess or the justice of the vigilante. He's the Heart.
There are other trios in the DCU that also come to mind, but they don't fit as well. There's the Young Justice founders - Tim, Kon, Bart. But Tim is more than the Head, and Kon was not quite Heart, and Bart was something other than Hands. And then there are the Bats - Bruce, Dick and Tim.
And Bruce might be the head, and Tim is his Hands in so many ways, but Dick - Dick has all three in balance, as befits an acrobat. It's why he's a natural leader and why it's a good thing Didio didn't kill him off - he's in many ways the Heart of the DCU - second only to Superman - but he balances it with a brain trained by Batman but lacking his obsession and a willingness to do what is necessary, tamed by a belief in all life.