The left-most (una corda) pedal can also be used to generate changes in tone colour, it isn't just for volume. I have been to recitals where the performer used the una corda but played at actually quite a high volume, and it was done just to change the tone colour. Teacher also told me that in certain performance situations, it could be used as an "emergency brake" to reduce the volume of the whole piece, for example performing on a very loud piano with a soloist who might be easily drowned out.
The sostenuto (middle) pedal is not always present on upright pianos- some uprights have 3 pedals but the middle one is a practise pedal that activates a muffling mechanism. The sostenuto, if it is there, allows the player to depress one note and have just that one note sustained. I have no idea how it works, and have only used it once.