In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

You might look at Gunther's book in regards to creating a data table and generating a sinewave.  You actually have to generate 3 sinewave [3 phase current] that are 120 degrees out of phase with each other.  

I suspect you can use the same data table for all three by starting sequentally.  

The real question is what is the 'bare minimum' of data to generate and appropriate sine wave?  If you have extra memory, you can have greater resolution.  Gunther show a way to use actually 1/4 of the sine wave as your data.  If useable, that will give you a lot more headroom for progaming goodies.  {I suppose that you can do some smoothing of the output with filter capacitors or maybe even a software filter of some sort.}

The back EMF is discussed at PIC's manufacture.  I believe it is measured at the expected 'zero crossing' as it is easiest to recognize the back EMF by polarity rather than as an offset from the expected voltage.   
{I suppose there is a comparitor somewhere in there.}

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