Sentence:  But when the moon rose and the breeze awakened, and the sedges stirred, and the cat's-paws raced across the moonlit ponds, and the far surf off Wonder Head intoned the hymn of the four winds, the <WSD>trinity</WSD>, earth and sky and water, became one thunderous symphony&mdash;a harmony of sound and colour silvered to a monochrome by the moon. 
Meaning of the word marked with <WSD> tags:  A group or set of three people or things; three things combined into one.