Endometriosis Support Group
Endometriosis is a common medical condition where the tissue lining the uterus is found outside of the uterus, typically affecting other organs in the pelvis. The condition can lead to serious health problems, primarily pain and infertility. A major symptom of endometriosis is pain, mostly in the lower abdomen, lower back, and pelvic area.
However, know that just because the material sampled didn't come back with endometriosis does not necessarily mean you don't or didn't ever have endo! Endo is notoriously difficult to recognize and consequently can be hard to biopsy. It could also be your last doctor got most if not all of it, that too would explain why none showed up.
You should of course talk to your doctor about what exactly was removed, but with three previous surgeries I am guess a lot of it was scar tissue and adhesions (which can be due to many things including previous surgeries or endo) in which case I wouldn't worry about it too much and it might still help you feel better as adhesions can be quite painful.
Endo is the muscle - this sounds like perhaps adenomyosis if he was talking about in your uterus?