Just spit balling here, but it seems to me maybe Elliot's personalities are compartmentalizations of Jung's model of the psyche. Elliot is the persona, Mr Robot is the ego that protects the persona... And the third personality is the shadow, which is why we've been (mostly) unaware of it. It's the darkest parts of Elliot, the parts he'd rather not look at maybe? His Monster?

I'm starting to wonder if Elliot's shadow/third personality is the man in the black hat that feeds Elliot cement in the waking nightmare of Elliot's Adderal abuse. We see him when Elliot is eating with Leon as well.

Has this character been in the background more than we realize? In the shadow of Elliot's identity? Pulling the strings of Elliot's psyche? Is Elliot sitting in his seat at the end of the episode?