For 3rd boss there are two types of adds that spawn, all dps who do not have the boss killing buff should make sure to focus these adds. The dps with boss killing debuff just stay on the boss.

The first type of add run straight for the pots and spawn where the boss is being tanked. The second type can be tanked by one of the many extra tanks at your disposal and tend to spawn on the outsides of the platform. I wasn't paying attention so I don't know how the tanks were holding threat when the boss is invulnerable to all who don't have the debuff, maybe flash and luck? If I had the chance to organize it I'd say keep one tank from each group on the boss ready to grab threat and hold it and let the extra 3 'off tanks' grab the tankable adds as they spawn.

I can only add extra observations about the 5th boss since I haven't been in a group that has killed him yet.
Two types of adds spawn in this fight, bombs and iron giants. The bombs will try to go toward a tower and blow up. I have no idea what happens if they destroy all towers like this because my group wouldn't try it. Supposedly comets destroy the towers if they land near them although I never saw this happen unless they blew up, in which case why not let the bombs do it?
The Iron giants have a large amount of hp and a crazy wide and powerful cleave, an off tank needs to pick this up and move it out of everyones way. During the entire fight as listed above, everyone is taking dmg, the dmg color on the floor changes from blue to green but I have no idea what this means when it does.
I would argue that if you are standing right next to a tower you do not actually take the floor dmg, this indicates that the best place to tank the boss would be facing center right next to a tower since he doesn't seem to have a rear facing cleave. No idea if this would work because my group would not do it.

It would be really great if someone who has actually killed king behemoth would jump in with something.

The reason you let the debuff stack to 2/3 (if you stack to three click it very fast) is because the more times you click the towers the more you charge king behemoths ability to use meteor which wipes the raid.