Erik's way is pretty good, though the harmonic content can be poor unless you are very careful.

Easiest way to get the sinewave ? Pump a clock driven filter chip, say fifth order or better with a clock say 10 x higher than the frequency of interest, drive the "input" with your square wave, and the output wil have a nice, clean sine wave. The triangle wave can be derived with other methods, the classic there is probably diode networks, but an integrator could also be controlled by the filter technique used for the sine wave.

And not an 8052 in sight....
Steve