Why Do Piano Treble Notes Often Use Three Strings?

Open a modern piano and the pattern is easy to see: the lowest notes often use one string, the lower and middle register moves through two, and much of the middle and treble range uses three strings for a single key. One hammer strikes this group of unison strings together. The extra strings certainly transfer more vibrational energy to the soundboard, helping the upper range balance the rest of the instrument, but they are not simply three identical copies.

The strings are tuned to the same pitch, yet their contact points, supports and actual motion cannot be perfectly identical. Those tiny differences affect how the strings interact after the attack, so the sound does not decay as simply as it would from one string alone. What we hear is not merely a louder note, but a fuller and more animated continuation. The Metropolitan Museum of Art notes that eighteenth-century pianos commonly had two unison strings per note; increasing this to three by the early nineteenth century formed part of the piano’s development towards greater power in larger public spaces.

This also explains why a tuner separates the strings of a unison with felt and adjusts them individually. If their pitches differ too much, obvious beats make the note sound unstable. When they are properly tuned, their slight differences become part of the colour. A pianist should not hear “three notes”, but one note whose clear attack opens, blends and dies away. Some of the piano’s richness lies in this carefully controlled lack of perfect sameness.

https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/mechanism004.html
https://www.metmuseum.org/essays/the-piano-viennese-instruments