I have been having issues with the charisma 3 thing, and looking at other monsters that have cha 3.
I noticed the war forged charger has 3 cha and it doesn't look like it came from the blue lagoon. lol
Warforged Charger's also have Int 5 and Wis 7, so a different explanation can be made for their lack of persuasiveness and personal magnetism - they are dumb as a brick. Deeply mentally handicapped persons are generally more socialable than warforged chargers. Also, warforged charger's happily live to the left of the uncanny valley. Being completely inhuman the minor animal-like resemblance make the otherwise horrific appearance of the construct look a little bit cute - but not in a way that you'd take orders from, more like how a bulldog, gorilla or a warthog can look cute. However, the more real the thing looked, the less attractive I think it would be. It's easy to avoid revulsion on the thing when its inanimate and cartoonish. If you met the thing IRL, and it tried to talk to you, I'm pretty sure that you'd find it disturbing and unlikable.
It sounds to me from your objection that you've taken the absolutely worst possible charisma a human can have, and are trying to advance an argument that his charisma isn't utterly crippling - he doesn't look that bad, he isn't that offensive, he isn't so unattactive, etc. While I'm sure equally powerful explanations for his three charisma can be advanced beyond a belching flatuant vulgar obnoxious drunkard, any of them that fail to convey a picture of equal social ineptitude don't really convey charisma of 3. This is someone who is brutally unlikeable in every respect, whose charisma is probably lower than just about any protagonist in any fiction I'm familiar with and certainly lower than any also not described as being low in wisdom and intelligence. The closest I can come to is Nobby Nobbs in Disqworld, whose pictures you should look up, but the thing is I'm not sure that I wouldn't stat Nobby at 4 or 5 charisma because he's not a completely unlikable person. My description is intended to fit with the worst traits of an aggressive drunkard.