Er, well there''s no remotely easy way of doing this. Here''s a few methods.

Alternate between the different frequencies. Easiest to do but has a strumming or wavering effect to it.

Calculate the FM synthesis of each, average them, then if it''s above a certain threshold turn the speaker on, below, turn it off. I''ve never heard this one implemented, so I have no idea of the quality.

Mixed harmonics. Beats me what it is, just heard of it. . . once. . . don''t know where. . . don''t even know if it actually exists.