There is another factor on why people are not going past the 3 inch very often.
Generally their partner asks them to stop. Also if you talk about the size spectrum from 5-7 inches adding 2 inches to either puts them in the 7-9 inch penis which honestly is more than enough to satisfy. So adding the 3 inch marks move them to the 8-10 inch mark which is exceptional well hung.
Even if you could gain another 3 inches, it amount of damage you would have to do to attain that, is pretty intense.
Also it focuses on an approach to target each system individually. Hence first would be the ligaments, then you would focus on the tunica, then focusing on regrowing veins to fill said new growth, and then focus on girth and repeat veins.
Also the human body force growth is very very slow process. People have had their legs broken and created more growth plates to get taller. That is a very slow process. While we focus on day to day, week to week, those small windows of time line isn’t enough to create progress, on the grand scale.
The next thing that stalls progress is we humans tend to over complicate everything and think more is better, and can’t seem to figure out what works and what doesn’t. So even if someone did attain the 3 inch mark, no body would be able to decipher what steps to take, and if you did it’s possible that doesn’t work for your body to respond.
I will give you an example, while a lot of people tout hanging huge amounts of weights and fatigue the ligaments and continue to progress and over load the body into more growth. I have gained the most in the 2-5 pound range. 3/4 of an inch in a months time. Not even setting a schedule and paying attention to how my body responds. But someone else would look at that and say that wouldn’t do a thing for me.
Granted I have 8 years of experience under my belt and I know what I should be looking for and made a lot of trial and errors in that period of time. I also wasn’t consistent the entire 8 years. I feel people with more experience also have a chance for more success, however I see the wheel being reinvented a lot in this subject. Even if that knowledge is passed on, people still explore other things, because humans are curious.
These are several reasons why the 3 inch mark is highly irregular to attain.