2 and 3 sound odd to me. "more or less" hints at an adjective/adverb, thus in the 1st it means "I know him more or less (well)" but in the 2nd and 3rd it doesn't work.

I've seen him round. I guess he's more or less known to me.

They are all acceptable. "More or less" isn't so much about an adjective or adverb here as it is describing 'how much'. It's a qualifier. You can say, "I'll be there at noon more or less." or "Tony's a good guy more or less." The first example means that I'll be arriving at noon, but there's the possibility that I'll be there a little before or even a little after. The second example means that Tony is not a bad person, but there are perhaps some things about him that are not so good.