split 2 shift into 3 shift and how to auto reset series of reference along w month changes

I added my encoder and need to split my autofill shift from currently 2 shifts into 3 shifts but i find it hard to do.
another thing is I am using an auto fill Reference from my table, reference pattern is 18-year, 12-month, 001-number series(1812001), how can I make number series from auto reset and back to zero when month changes
see attached file