Poetry frequently gathers different human conditions under one common consequence. The poetic example names “youth, leisure, and prosperity” as three distinct conditions.
Their significance lies in the shared outcome attributed to their conjunction. The figure does not require youth, leisure, and prosperity to be the same thing in reality; it requires the speaker to organize them around one relevant meaning.
Structure:
Plural referents — youth, leisure, prosperity.
Common relation — a particular result associated with their conjunction.
The rhetorical point is the concentrated connection, not merely the fact that three nouns occur in one line.