This seems to be causing confusion at present, I have just answered this elsewhere.
When wound as Ed showed it, the coil displacement in the 12/36 is at 60 deg.
If the starts were taken from alternate coils and not the 3 adjacent ones the displacement would be 120 deg and the star point would be formed by joining the 3 starts( or finishes)
With the 60 deg displacement, phase 1 & 3 are at 120 deg and the middle phase can be corrected by reversing its start and finish. If you draw vector diagrams you can see why it works out, otherwise just accept it.
If the 3 phase winding is based on the alternative single phase with half the coils then this ambiguity can't arise, so the 12/18 version would not require the middle phase to be reversed.
It goes back to the single phase issue, with the same number of coils as magnets you must reverse alternate coils S F F S S F. If you leave out the intermediate coils then yo connect the remainder S F S F.
When wound as a single layer winding of the 12/9 type there is no need to reverse any coil.
Flux