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

Thanks BO_Ry, the first formula works well, however the second one doesn't work.
but still Im so thankful for this it really helps alot to me, maybe you have an idea on how can I split the shifting sched. from 2 shift making it to 3.
below is the formula I used for 2 shift:
=IF(G2="","",IF(AND(TEXT(G2,"HH:MM")>="06:00",TEXT(G2,"HH:MM")<"18:00"),"Shift 1","Shift 2"))

thanks a lot Bo_Ry, it helps and it work.