Canon perpetuus

A short three-voice canon

From Wikipedia:

A round (also called a perpetual canon [canon perpetuus] or infinite canon) is a musical composition […] in which a minimum of three voices sing exactly the same melody at the unison (and may continue repeating it indefinitely), but with each voice beginning at different times so that different parts of the melody coincide in the different voices, but nevertheless fit harmoniously together.

The score is available as PDF.