I think it can be done. I think the DTMFout command uses it. SOUND will not work for polyphony.

You definitely do not need 3 buzzers. But a lot of quality coding(highly optimized asm).
Then, you could play out samples(not just square waves of sound) which could be n-key polyphonic thru a single buzzer/speaker. Interrupts may give determinance of time between samples, but it may be hard to get the timing right.

You might like to read up on this for ideas even though it uses the PSoC
http://www.web-ee.com/Schematics/PolyPiano/piano.htm

Another using the PIC is a DDS
http://www.min.at/prinz/oe1rib/DDS/SineDDS.htm