The abilities of Haldan and Damrod make a great combination. The main strategy of this deck is to continuously throw attachments into the staging area at enemies and locations, receiving the card draw and attachment benefits.
The choice of third hero is pretty open. In fact, Haldan and Damrod by themselves make a pretty good secrecy deck and allow you to contribute in multiplayer early with all the attachments while you get setup, which can be difficult to do with many secrecy decks that need a few rounds to get going.
I’ve chosen Haldir of Lórien to round out the deck in this case, because its fun to put Glamdring on him and snipe enemies that are either trapped or engaged with other players. Together with the card draw effects from Haldan and Damrod, getting another card with Haldir of Lórien + Glamdring usually means you can easily draw 3-5 cards each round, fueling the core strategy for a long time.
A nice benefit of having attachments targeting both enemies and locations is that Guarded attachments are easier to claim (assuming I understand the mechanics and timing of these things!). For example, playing Poisoned Stakes into the staging area followed by Glamdring or Necklace of Girion means that if an enemy is added as the Guard it also gets trapped in the Stakes, and then you can just wait for them to die to grab the Guarded attachment. If they reveal a location, you can play Woodmen's Path, Woodmen's Clearing, Elf-stone, etc. after to make it easier to clear or get other bonuses and then you still have a trap ready in the staging area.
There isn't much defense from your starting heroes so getting Treebeard out as your primary defender is a top priority. If defense is a problem, a different third hero besides Haldir could easily be swapped in for more defensive options to start. Other Ent allies are there to put stats on the board. Master of the Forge is clearly ridiculously good in this deck with all the attachments. The sideboard has other cards that I think would function well in the deck but didnt make the cut for this general multiplayer version.
Haldan and Damrod make a great combo - I think it will get even better as the cycle advances and there is a lot of flexibility in third hero options to optimize the deck for specific scenarios.