They're all VERY usable in OU. Hydreigon got Superpower, Roost, Dark Pulse, Earth Power, and more from the tutors. Superpower means the fat bitches cannot just come in and do what they want. Whores.
Nidoking just got Sheer Force from the Dream World and he has a ton of moves to abuse with a Life Orb.
Scrafty gets Dragon Dance. With its great bulk and good options for STAB moves, it can do pretty well. He was OU till the last tier change a month ago, but I have never used him competitively so I dont know all his options.
i don't quite agree with the idea of them all being OU. only hydreigon is solid OU material, the others both suffer from a serious lack of offensive presence. i will try and follow up to be clearer.
hydreigon has always had its niche in ou. with its infuriating speed tier (98 >_> cmon gamefreak, what's with all the mons between 95 and 100 this gen), it has a VERY tough time being a sweeper, esp since it gets no speed boosting moves a la dragon dance. less than 100 base speed just doesn't cut it anymore, unless you carry a scarf. as for scarf hydreigon, it works, but i've always preferred it as a wallbreaker because of its preposterous coverage and strong mixed attacking stats. hydreigon's coverage is markedly superior to latios's (fire blast/earth power/superpower/dark pulse are all good options), and the two are the best special attacking dragons in OU - latios's advantage ofc is its much superior speed. that tends to give hydreigon a great niche as a wallbreaker and imo it plays best with life orb/ebelt so that it can switch moves and abuse the coverage to dismantle defensive cores (MANY of which fall to hydreigon's coverage). on top of that, as gvon mentioned, it got superpower, which, backed by hydreigon's completely usable attack stat, will make a mess of blissey/chansey. hydreigon's poor speed means it won't be sweeping easily, but very very few things can claim to wall or hard-counter it. on top of that, hydreigon is one of the few dragons that gets uturn and/or levitate, and we already know how useful they are
nidoking... sadly nidoking just doesn't have ALL the stuff it needs to be OU. sheer force was a great boost for it because of its very good earth power/sludge wave/fthrow/ice beam/tbolt coverage (yes, i am aware that's five moves). all of those moves get sheer force boosts, which to some extent compensates for nidoking's lack of raw power and, except earth power, lack of stab (iirc, sludge wave is mainly to hit rotom-h neutrally in UU; otherwise it's not a great option). plus sheer force will block nidoking's life orb recoil on any of those moves, which lets you stack your way to a boost of about +70% (maybe +60% if the boosts are additive instead of multiplicative, idk what they are for sure). the problem is that nidoking's spatk and speed just... they just aren't that high. great coverage and sheer force boosts aren't enough for OU, when you've got base 85 spatk and 85 speed. it also has a niche in its typing, which resists terrakion's stab combo (but does not resist edgequake). however nidoqueen is bulkier so i think it would do that job better. either way, neither of them are straight up OU material. there are other mons in OU that can pull off a broadly effective combination (eg ice/elec/fight from stuff like thundy-t) but they have more speed and more spatk. it's tough to find an advantage for nidoking.
scrafty is one of my absolute favorite mons but the tier drop to UU should be sending some warnings about its OU viability. what does it have going for it? firstly, a great stab combo. fighting is a great stab right now, and if you add dark, toxicroak and heracross are the only resists. secondly, three FABULOUS abilities. shed skin gives it insurance against burn and lets it use rest semi-viably, and we all know how good moxie and intimidate are. and third, significant natural bulk thanks to 65/115/115 defenses. on top of that, scrafty has bulk up, and freaking dragon dance. the problem with scrafty is similar to the problem with nidoking though: 90 attack and 58 speed? you're not about to wipe up OU. scrafty has a max speed of 236. at +1 you can't outspeed starmie, and you're slower than freaking tyranitar, who with 61 base speed is among the slowest attackers in OU. you'd think scarf could get around that, but scrafty doesn't have the firepower to abuse a scarf - when it comes in, it doesn't hit hard enough. bulk up has its niche with scrafty's great defenses and access to rest+shed skin, but bulk up sets invest heavily in bulk, meaning scrafty needs several boosts to bring its subpar attack to a useful level - that's the same problem as dragon dance. ultimately that's its weakness: scrafty presents no offensive presence without a boost, and it tends to need multiple boosts to become a threat. with so many mons in OU that can boost, and yet don't need a boost to pose a threat, why run scrafty?
anyway yeah all 3 mons have their place, and don't get me wrong i have personal love for all of them, but only one can truly claim to belong in OU
EDIT: @below: porygon2 sounds like a very very interesting counter to tornadust. superpower is a 2hko not counting attack drops, so after two superpowers 0atk LO tornadus deals max 82%ish - not even a ko with stealth rock. 252 LO hurricane narrowly hits 50% so you have less than 1% chance to get 2hkod. 252 LO focus blast deals min 56% though, so it can 2hko (assuming it hits lol). plus you trace regenerator as you come in so you can always gtfo if you don't feel like sticking around. porygon2's 252+ ice beam vs 0- tornadus t (ie naive versions, which seems to be the norm when running superpower) is min 79% which is a ko after stealth rock and 2hko all the time. plus it's not damage that tornadus will be able to laugh off by regenerating since that still leaves like half its HP missing the next time it comes in. alternately, you can grab a download boost on the way in if tornadus-t is naive (194 sdef vs 196 def), which means your 252+ ice beam will definitely ohko and you pose a significant offensive threat after tornadus dies. (aside: uninvested ice beam still hits tornadus t for min 58%, and uninvested +1 ice beam is min87% with 1/8th chance to ohko). as far as the definition of counter goes, porygon2 is very solid. can't speak to its viability beyond that though because i haven't used it myself