Personally, I prefer #3: "The only times I heard French slang, it was in the dubbing of English films."Originally Posted by navi tasan
I'm not sure that any of them imply that you were doing the dubbing. One variation could be "The only times I heard French slang, it was whilst I was dubbing English films."
Also, I would use I've in the first half... "The only times I've heard French slang,..."