Right (Sustain/Damper) - Removes the dampers from all strings allows them to vibrate freely. This serves two purposes. It assists the pianist to play smoothly connected notes. Pressing/Raising the damper pedal also causes all the strings to vibrate sympathetically with whichever note is being played, enriching the piano's tone.
Middle (Sostenuto) - Sustains only those notes that are depressed at the same time that the pedal is depressed, allowing future notes played to be unaffected.
Left (Soft/Una Corda) - Shifts the whole action including the keyboard slightly to the right, so that hammers that normally strike all three strings for a note strike only two of them. This softens the note and also modifies its tone quality.