I'm hoping to get feedback as well as opinions on this so I can look into it further and understand this process complete.

My current understanding for DC, is simply polling frequently and recording the DC voltage, and DC current. Multiply the two together and you have your answer.

AC on the other hand is a little more complex, and this is where I have a bit of a hole in my understanding.

Polling the AC voltage and Current must be done at a higher frequency than the AC supply, or you could accidentally poll during the peak or zero crossing... giving you incorrect results. That said, if I rectified the output for the voltage and current readings, and averaged it with hardware... (ie, capacitor.) would I still get accurate results?

This starts to get much more complicated when you move from a single phase supply to 3-phase... I'm not sure where to start at all with a 3Phase setup, and tips and information are greatly appreciated.

So.. tl;dr

I'm after formulas and information regarding measuring single and 3phase supplies, as well as pros and cons to taking a larger number high frequency polls, or averaging the readings with analogue electronics.