How does a triple fugue use three subjects?

A triple fugue is a musical composition that utilizes three distinct subjects, which are treated both separately and simultaneously throughout the piece.1

In practice, composers often provide each of the three subjects with its own complete, separate exposition. These expositions may even occur in different meters. As the fugue progresses, the composer brings the subjects together, combining them to create a complex counterpoint. For example, in J.S. Bach’s St. Anne fugue, the first subject is eventually combined with each of the other two subjects.2