Still tinkering around with this deck list, and still need to run it through a campaign. The idea for this deck is to support a group through heavy defense and offense. Once all engines are online and full throttle, the deck will quest strongly as well.
Denethor's role is resource support and defense. Because he starts off with two additional resources, you get early access to cards you normally wouldn't (e.g., Knight of the White Tower on turn two rather than turn four; turn 1 if you have Steward of Gondor in hand). Additionally, his ability allows you to move one of his resources to another Gondor hero. So if you need an addition token in the department, you can add a resource to Beregond's pool. Moreover, if you are planning with other people, you can play In Service of the Steward on one of their heroes to give them one addition resource if needed. Because of these tactics, you will almost always want to put Steward of Gondor on him. This will also allow you to put Rod of the Steward on him to draw cards with excess resources (also thematic). Lastly, you can equip Denethor with Armored Destrier and Gondorian Shield to make him a very respectable defender.
Boromir has a nifty buff for all Gondorian allies (not just yours, by the way). It is essential for him to have Visionary Leadership as soon as possible. That way, when you have at least one resource on him (which can be added through Denethor if you somehow run out) all Gondorian allies get an additional and (this also includes allies belonging to other players; and if you put In Service of the Steward on one or more of their allies, you can give helpful boosts to cards that normally wouldn't get them). Most of the characters in this deck lack strong for questing, which is the major weak point. Faramir is a classic help for this as well. If you put Heir of Mardil on Boromir, then you can ready him by having his dad place a resource on him after his first attack. This will enable him to attack again. Additionally, he is the prime target for Orcrist and Valiant Sword to boost his attack drastically.
Beregond is a wall for all players. Having sentinel enables you to block for yourself as well as others. 4 is nothing to laugh at. Pair him with Gondorian Shield and he goes to 6 immediately. Additionally, because he has sentinel, you can put Armored Destrier on him, allowing him to block multiple times. Also, he is the prime target for Behind Strong Walls when there are many enemies engaged with you and others (readying him and allowing him to block multiple times) and Sterner than Steel (making sure bad shadow effects are mitigated).