'Project X' Star Jonathan Daniel Brown -- I Once Banged 3 Chicks in a REAL PORNO!

He looks vaguely familiar ... but you can't place exactly where the hell you saw the nerdy guy in glasses from "Project X" before -- well here's a hint ... he banged three smoking hot chicks in a popular porno you watched.

His name's Jonathan Daniel Brown and he plays JB in the new Todd Phillips movie -- his first big acting gig -- but TMZ has learned, he made his on-screen debut about three years ago ... in a slightly less popular title ... "Nerd Hunting," a Bang Bros. production.

Sources close to Jonathan tell TMZ he wasn't gunning for a career in porn at the time -- he was just looking for some first time on-screen experience ... "It was just kids being kids."

Nothing to be ashamed about either ... the girls in the porn flick are ridiculously hot -- and we're told JB's still got the magic touch, attracting serious female attention ever since the studio began screening "Project X" ... a movie that's already been hailed as an instant classic. Plus it did well in midnight showings, so a return to porn won't be needed anytime soon.

So there you have it -- Jonathan Brown ... the nerd you wish you could be.