Priamond, Dyamond, and, their brother Triamond are pretty much the ultimate trio. Their (awesome!) names each locate them within this model of three (Pri - suggesting one, Dy - suggesting two, and Tri - suggesting three) and suggest that especially for the first two brothers, their identities are in many ways inseparable.
Not only are these guys extremely close brothers, but their mother Agape has actually ensured that their very lives—literally their life-energy—are also interconnected. When Priamond dies, his strength passes to Dyamond, and when Dyamond dies his strength passes to Triamond.
Since Triamond ultimately lives, he truly carries his brothers with him. This deep, spiritual connection between the three brothers that associates them with one another even after death makes sense for Book 4, which is all about friendship and types of bonds. Even before Triamond meets Cambell, these three men demonstrate the power of fraternal friendship and the bond created by family. They're bros for life… even after two of them are dead.