Having beaten the Cave of Ordeals more than 10 times with relative ease, I think I can give you a few ways to dealing with them. Pick the one that suits you best! I'm not the best player, though, so maybe these techniques might not work for you. ^_^;
First, some tips. Use Great Fairy's Tears, Rare Chu Jelly, or if those two aren't available, Blue Chu Jelly. These heal all of your hearts no matter what. Make sure you watch your health. Fairies aren't recommended since they only heal eight hearts. It's a little more convenient, but in this case, it's not worth it. Also, the Magic Armor is a big help. Fairy's Tears/Rare Chu Jelly + Magic Armor gives you a temporary powered-up Great Spin Attack and invincibility!
The Back Slice is a good technique to use as well, especially for the second stage of the Darknut. More on that later--
1.) Here's the one I use. I pick them off one by one. No matter what you do, the three Darknuts will eventually merge into one cluttered little group that's annoying to deal with. So, I target the one in the front. Right before the Darknut strikes, I backflip out of the way and Jump Attack him (but if I'm a little too close, then I use a regular jab attack combo). Two regular sword slashes + Spin Attack + a final regular sword slash is all that you can cram in the time he's stunned. Right afterward, a side jump + a small slash during that jump followed by a Black Slice works nice as well. It rips off a nice amount of armor, and the Darknuts in the back might get some of their armor stripped as well after this combo. While you're doing this, make sure that you're watching the other two Darknuts carefully, as they might strike at you during your combo. Once you're finished breaking all of one's armor, run towards the Darknut really fast while it's still away from the others. It takes two complete regular sword combos to defeat the stripped Darknut. For me, it takes less than a minute to take care off -- Black Slice, combo. Black Slice, combo. Done. Repeat with the others and heal if needed!
2.) When I'm feeling lazy, I use this method. XD I stand from the ledge and Bomb Arrow them 'til their armor is gone. Note that the Darknuts eventually all gather around underneath the ledge. At this point, use regular Bombs and drop them into the fray. Be careful at this point -- you'll have to listen to the final clang that the Darknut makes when all of its armor is gone. When it throws its sword at you, it can still hit you
through the ledge and make you fall down! Listen to the warning sound and dodge. Once all of their armor is stripped, you can fall down and begin your massacre, but I have to admit that I've come to dislike this method after a few more rounds of the Cave since three un-armored Darknuts are, to me, harder to deal with than one of them and two armored Darknuts. Either way, you can use the Jump Attack to stun them, or the Back Slice. Take note that if you use the Back Slice, you're more than likely to get hit by the other two Darknuts midway through your roll or jump. :|
Another thing to mention -- if you somehow fall down from the ledge before you're finished stripping their armor with Bombs, you still have a chance. Use regular Bombs (not Bomb Arrows) and throw them into the group of Darknuts. Walk backwards so that they'll follow you. Make sure that when you throw the Bomb, it's at a distance where by the time it's ready to explode, it's behind the Darknut. Because of this, you might want to throw the Bomb in front of the group before walking backwards to lure them into the explosion.
3.) This method requires a good amount of reflexes. Watch for the Darknuts' attacks. When they attack, Shield Attack them back. If successful, you can use the Helm Splitter on them and crack their nonexistent skulls. :3 Slash at their backs. Once they're stripped down to their chainmail, watch their attacks again. When they jump forward and do a double-slash at you, jab at them from a distance right when they draw their blade back. You can get four hits in. Repeat.
Some other things I'd like to say!
If you somehow end up in the middle of the Darknut cluster with the intention of only trying to hurt one, more than likely the other two will try to hit you. When you see it coming, do a Spin Attack really fast to make them bring up their shield. This way, you
might get a hit in with the Darknut you were facing, and the other Darknuts can't attack for a little while. I suggest that you get out of that cluster, though. ^_^;
From what I remember, the Mortal Draw can also be used against these guys, although it's a much slower way of dealing with them.
If you're having trouble picking off an un-armored Darknut from the slow ones, throw something at it (a Bomb, maybe, or you could Clawshot it) and it'll dash out of the way. This will give you a little bit of time to rush in and fight it before its buddies come in to bully you.
Oh! Remember to stay calm. :3 Good luck!