Bruce usually talks about the 3 tan tiens. Once in a while he mentions the 3 guas. I think gua just means section, but I’m not sure: I don’t speak Chinese. The three refers to the lower, middle, and upper. At this point I think you are supposed to open up (gain access and control) of the guas before the tan tiens. In my practice I’ve found that the diaphragm and guts act like a sphere. The brain ventricles act like a sphere. And, the lungs rib cage act like a sphere. That’s my understanding of the lower, upper, and middle guas respectively. The tan tiens are deeper and on the central canal. Everyone should know approximately where lower tan tien is. The upper tan tien is in the 3rd brain ventricle and the middle tan tien is right at the heart level.
I found postures 9 through 12 to be incredibly helpful for opening each gua and finally to be able to work all three simultaneously. Once each sphere is open, accessible, and under your control – each of the tan tiens will start to make sense.