If you go with just 2 drakes, kill shardron it will give more time to deal with tenebron and his adds + you wont have that +100% fire damage debuff.

But if your dps pulls really 4.2k dps on 10man fight were you need to move like hell go 3 drakes, our dps did like 1k less and we have killed her 3 times with 3 drakes up.