That's what I'm good at.

"That's where I'm good at" is wrong, because "where" can't be used with "at" in standard grammar. Even "that's where I'm good" sounds strange to me, really, but I guess it could be used in certain (rare) situations.

If your good at it in a certain situation, you say: That's where I'm good (at). [where = situation] If your good at something, you say: That's what I'm good at. [what = something] OK?

The first phrase could be acceptable depending on the context. The second one, " that's what I'm good at", is most commonly used. The third one is incorrect.

That's where I'm good at is correct if talking about a certain department, area or place. That's what I'm good at is correct if talking about the job or action you are doing. That's my good at is not correct.