Sue
I seem to recall that term being used as well.

One of the nurses at my clinic also called it the leaf shape.

I think the triple line may refer to how the endometrium looks on the ultrasound. The three lines refer to the lining on the walls of the uterus and the infitessimal gap between. Sort of the two slices of buttered bread thing, only leaf shaped or with the appearance of three lines.

Not sure though... could just be one of my imaginary theories.