Before phase 3,stop all dots,use any threat drops you can and who ever gets aggro in p3 heal them and make sure they go to the tank spot.Wait until tank picks up aggro again then start nuking.
Usually our fury warr gets aggro in p3 and we just keep him up until tanks gets aggro on Onyxia.
What my guild does is have 3-4 tanks in the group, MT will take ony on pull. In P1 the other tanks will try to be around 2nd-12th on the threat metres. This means that going into P3, some tanks will still have plenty of threat on the boss.
We have no idea how the threat reset actually works, but it’s definitely not a hard threat reset on all players. We think the fireball she does resets it.
Killed her on this reset without any such problems. In p1 noone goes mad and starts autoattacking after 5 sunders and proper positioning, in p2 everyone blows everything, downs her to 45-43% and stops dpsing altogether. Tank grabs her right on landing and positions before first fear without any issue. On the whole 2 tanks had very steady threat. Fireball did not reset threat, specifically looked for that when it hit me (if tiny threat is to be trusted on the matter). Did this with full raid, only 3 ppl died.
Killed her today, as far as i know her Fireball in P2 resets all threat, but there is no reset from p2 to p3.
Our guild struggles with her, as we moved from 40 man to 20 man splits on her.
Maybe it was just luck today, but what we did today worked quite well.
Casters wanding P1 & melee doing most Dmg
Transition to p2 all dps as much as possible, ranges do as much dps as possible.
Meanwhile the 2. tank tries be just below the threat of the melee dps.
On p3 all who can reset, reset aggro, 2. tank use all dps cds and get threat asap.
Worked well, he didnt got hit by fireball
Thats why using gnomish cloaking device makes the whole transition trivial because it gives everyone an aggro reset
Threat addons are absolutely not to be trusted. Actual threat tables aren’t accessible to them in Classic, they’re just parsing combat log information.
Aggro does not reset in P3. All the DPS are high on the aggro list while tanks can’t taunt or DPS while she is up there so she takes turns according to highest on the aggro list. What we did tonight was have all tanks fight for aggro from P1 and then when she landed, she went for one of them. Worked like a charm and only one death. It was a hunter though so it doesn’t count anyway.
That’s what i wrote in one of my posts earlier when i wasn’t taken any serious on it :
#2 - Use 3 Tanks , not 1 and all of them should stay at the front of Onyxia and eat Cleaves for more Rage = more Threat (all DPS should stay under all 2-3 Tanks threat, recommended 3 tanks).
Of course they can. Sunders are laid like a charm.
To my understanding, the best way to manage threat is:
P1: only melee dps, offtank make sure he is second in threat until P2
P2: only caster/ranged dps, noone from P1 dpsing ony.
P3: offtank should be among the top in threat.
With this method, everyone including offtank remain at even threat come P3. Tested in my brothers guild and it worked pretty well, can’t confirm though.
So much wrong info in here it’s nuts lol. The fireballs knock off huge amounts of threat. the MT can literally get 4/5 when she’s in the air clearing all threat. Then when she lands she cycles through the top 5 threat. Which means it’s a good 30 secs for MT to ger her back.
If someone stacks with MT for fireballs, it doesn’t clear his threat. So when she lands she’ll cycle top 5 threat, then go back onto the MT. So ideally you want all wars to be top threat, and to loosely stack where you position her.
Or if you have good DPS can burn her quick p2 before any fireballs, she’ll be glued to the tank rest of fight then.
Wait until you become convinced she is breathing more often.
You just need your ranged DPS to run north, some will die but eventually you will get a tank have aggro and voala.
or gnomish cloaking device
Oh my god shut up about the cloaking device. You cannot seriously expect every single raider in the world to take engineering.
Nevermind him, he’s a lvl 1 troll 
Have the tanks always be first in threat at the end of Phase 1.
Have everyone else at max 80% of the last tank threat at the end of phase 1.
Phase 2 : stop all dots before Onyxia lands.
Landing in P3 : Onyxia targets, in order, all the targets that were top threat at the end of Phase 1.
As long as you have 2-3 tanks first in threat at the end of Phase 1, and everyone else 80% or less that last tank threat…there is no problem.
Onyxia has never, ever, through history reset threat.
The fireballs also dont reset threat.
Its just as hard as its supposed to be.
Use more than one tank.
If your guild cant figure out how onyxia works, good luck on later raids! 
Also, everyone who is trying to figure out new tactics for ony… dont try to reinvent the wheel.
If your standard tactics dont work, someone is doing something wrong.
Onyxia fireballs the highest threat targets on her list in P2. This wipes all threat from that target.
We’ve been having two designated tanks try to build up threat as much as possible in Phase 2 and on the landing but still have difficulty with it going on a tank despite not nuking her during transition.
We just get the people she targets to go stand in the tank position while we nuke her now 