Where will you get started and ended from?
If your run time is short, plain emonLib won’t give you accurate data, especially as your power factor will be some way from unity. I suggest you use the 3-phase sketch for the emonTx and choose the options for it to run on the emonTx Shield. This monitors continuously, even though it reports every n seconds, and as it will be calculating real power, it should come much closer to your supplier’s meter readings.
Because it’s reading continuously, it should be possible (if the code you add is fast enough and your reporting interval short enough) to also count the number of starts (i.e. the sum of the three powers changes from below some threshold to above) and report that along with the power.
If you’re thinking of a VFD, read this thread: Measuring AC current output from a VFD - #31 by Natashaxh