I can understand all types, including 3 since as 9 I both integrate to it, as well as having it in my tritype.
9s don't necessarily consciously have low self-esteem (this has to do with mental/emotional health levels rather than E-type; all E-types can feel bad abt themselves in their own way). Rather, other people seem to have firmer preferences where we do not, and we're so focused on others that, when we're around them, we want them to be happy and relaxed so they do not reject us to our face AND so they'll leave us alone from their demands and conflicts. The less healthy the 9, the more they will focus on maintaining surface (even false) harmony, to the point of numbing themselves. As I become healthier, I tend to actively seek conflict if the result is genuine harmony down the line.
E3 for me is simply an intuitive awareness of how others perceive me, and knowing how to dress /behave /etc so that they perceive me in a particular way. I never confuse real me with the image, which can be tweaked to fit in with whichever crowd while I am the real person, observing and often enjoying my "performance". Since I'm sx-dom, my E3 aspect is in sx-realm as well, not so much focused on work as, how good am I at flirting, getting attention whenever I'm interested, call-backs, etc, but still focused on "results, skills, and competence". I am so happy it's not my dominant type as it can really be a lot of work, and the sense of failure you feel when you "didn't achieve the objective" orz