You can sometimes "splash" a third, playing two colors as your base and a couple powerful cards or additional costs of another color, but I'd really focus on the two colors you want here
Aurelia, the Warleader Aurelia is the angel of Boros aggro, and Aurelia, the Warleader is the prime example of this
My plan is to let Urza's Saga get its third Lore counter, then tap it to create a Construct token, then Ghost Quarter the same Urza's Saga, so that during my main phase I make a Construct token and fetch onto the battlefield both a basic land and a Pithing Needle
Me, Id still run all copies of Nazgl that I can put in the deck, because honestly, when are you ever going to have all nine of them along with Witch-king on the battlefield
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