Hi! I'm grade 4 piano at the moment, and the only pedal I've ever used in my pieces in the one on the right which makes the notes longer, if you know what I mean, is it the sustaining pedal or something....? Anyway...
On some piano's there are 3 pedals, and on other pianos there are 2 pedals. I only know what the pedal I've been using does, so please could somebody describe what the other pedal/s do please? Thank you! xxxxx

The Wikipedia article here has a nice description of the pedals.

(In the 18th century, some pianos used levers pressed upward by the player's knee instead of pedals.)

wow! That is so cool! Being a very tall person, sometimes the pedals on pianos are really hard to play (such as the one my teacher has) and having "knee pedals" would so solve my problems!

NM

The problem with knee pedals is that their operation was very very clumsy - you'd have to lift your whole upper leg upwards to use the pedal, rather than just flicking your foot up and down using your ankle bone.

The clumsiness of the sustain mechanism is actually reflected in the clumsiness of earlier pedal notation, e.g:

Ped * Ped *

As the composers couldn't demand too rapid changing of the pedal, the notation didn't bother them. Once foot-pedals came out, the notation changed over a few years to:

P ____^_______|

I'm still not sure I understand the sostenuto pedal (my piano has a mute one instead). Is it like the sustain pedal, but without the 'sympathetic' vibrations of the other strings in the piano?

If you had to hold your leg up for a long time, wouldn't it start to hurt maybe cramp even? Ouch.

The normal sustain pedal keeps all the dampers up; the sostenuto pedal only keeps up the dampers which were raised the moment the pedal is pressed down. A good example of where it can be used effectively is the ubiquitous Rach C# minor prelude - a C# octave is played in the bass with a series of legato chords above it in succession. Play the C# octave, then depress the sostenuto pedal; the sustain pedal can then be used to keep the joins between the upper chords smooth. In the absence of a sustain pedal, half-pedalling can be used to achieve roughly the same effect.

As you guessed, you don't get sympathetic resonance as not all the dampers are raised.