> The cheapest hardware would be a dsPIC but you'd have to
> write firmware for the Goertzel or Fourier algorithms which
> would take some effort to get correct.

Microchip has some libraries, which might not be too painful, but if you
are looking for a specific frequency, there are some pretty simple cheats
that can work almost as well.  Three specific frequencies means just do it
three times.  The assumption is that you have 3 more or less sine waves. 
If you have some other waveform then you need to look to more robust
approaches.

Personally, I would tend toward a software solution, but 3 567's would be
the more hardware guy approach.  If the signals are more or less constant
this could work out pretty well.  If they go on and off, though, be sure
to avoid getting blindsided by the PLL lock and unlock times.

--McD


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