Piano Pages
What is the third pedal on a three pedal piano for?
On grand pianos this is usually the sostenuto pedal (the middle pedal). Notes which are already depressed and have their dampers lifted will remain sounding with their dampers lifted, when the pedal is applied, even if the notes are released. The remaining notes will not be affected.
On upright pianos the third (middle) pedal is usually a celeste or practise pedal that introduces a strip of felt between the hammers and the strings. On many older instruments with two pedals this is what the left pedal does.
On some smaller grands the middle pedal may lift just the bass dampers.