1) 習った言葉をきのう忘れた。 I think that would be the least ambiguous, with the least changes in word choice.
2) きのう習った言葉を忘れた。
3) きのう言葉を習ったことを忘れた。
Those are my guesses, but 3 is still a bit ambiguous, I believe. I left out 私は because I assume it's clear you're talking about yourself.