Regarding the OP’s piano, the celeste, or practice pedal bar is most likely bent slightly on the right side, as the left side is working fine……
The middle pedal on a grand is called a sostenuto.
Jean Louis Boisselot invented the sostenuto pedal 1844 and this invention was improved by Steinway in 1874.
On some grands( Stuart & Sons, Fazioli) there is a 4th pedal that moves the hammer set in a grand action closer to the string set, much like the soft pedal of an upright.
Sometimes in uprights and grand’s the middle pedal opens the bass damper set as a whole section while leaving the dampers applied to the tenor/treble.
In some high-end uprights the middle pedal is a sostenuto.