I'm not sure if this is true of electric pianos, but acoustic grands usually have the following pedals: left = sustain, middle = sustain for specific notes (more on that below), right = una corda. The middle pedal sustains whatever notes are depressed at the time that the pedal is depressed. So if at a particular time you have the notes A and E pressed down, and you depress the middle pedal, the A and E will continue to sound until you release the pedal, while all other subsequent notes will sound normally (ie, without sustain). This is useful if you have a long held note and other stuff going on simultaneously, which would make physically holding the note down difficult or impossible. Unfortunately, in practice, it's often tricky to "catch" only the desired note(s) - I don't tend to use it much myself.

The una corda pedal works by physically shifting the keyboard and hammers slightly to the right. If you've looked in a piano, all but the lowest bass notes actually consist of two or three strings struck at once. The una corda ("one string") pedal moves the hammers so that they strike one fewer string than usual. This produces a quieter and fuzzier/more diffuse tone. To understand why it's fuzzier, realize that in normal playing position, the felt on the hammers is compacted into grooves where the hammers strike the strings. While using una corda, the normally unused (and hence not compacted) part of the felt hits the strings, yielding the tone quality I described. This pedal is useful for color changes, and when you're trying to play very softly.

So that what the other two pedals do on a "real" piano. I don't know for sure, but I'd guess that an electric piano would try to simulate these functions.