all three are weird

Hi Pelin 🙋🏻‍♀️ I agree with Josh, "He's a childhood friend." is a great option. I'd also suggest trying "He's been a friend of mine since childhood." While the third works, "of mine" is a nice and common way to use the phrase as well.

Out of those, I would say "He's a friend from childhood ". You can also say "He's a childhood friend".