If one says "I don't know" in 3 different ways, like, when they stress "I", "don't" and "know" respectively. How does the meaning of this sentence change?
When you put a stress on "I" in "I don't know," you're actually making a distinction between who you are talking about: "I" don't know! (not him, her, etc.)
When "don't" is stressed, it is meant to emphasize the negativity of the sentence: I "don't" know! (in contrast with "I know!").
And finally when you put a stress on the verb "know," you're emphasizing what you don't do: I don't "know"! (in contrast with, say, I don't "want" or I don't "like"!)
I don’t know. Someone else knows it. It’s true that somebody knows it, but I am not that person.
I don’t know. That’s not true at all. Somebody has accused me and I’m protesting my innocence.
I don’t know. I only can suggest a possibility. In some way, I hint that, but I don’t have exact knowledge.