What are 3 pieces of evidence that the narrator in "The Tell-Tale Heart" is not insane?
Evidence there might be, but the evidence pointing to the narrator's insanity is more reliable. Still, that's not the question you asked, so let's see what we can make of it.
First, while we can't put much stock in the character's own assertions of sanity, his rational does make some sense. "Madmen know nothing," he says, going on to describe in detail the preparations he made for doing away with the old codger. He has a point. His ability to calculate and plan could be seen as evidence that he is not insane.
Secondly, he is able to understand right and wrong. This is very important, as it is often the crux of an insanity defense. He knows that what he is doing is wrong because he takes care to dismember and hide the body. Were he truly insane, the concept of right and wrong would elude him and he wouldn't go through such efforts to avoid detection.
Thirdly, guilt. The level of guilt he feels is enough to cause him to confess at the end of the story, even though he has every chance of getting away with the crime. Guilt shows a certain level of conscience, and this is not a hallmark of the insane.
I suppose it depends a lot on the definition of insanity. Most of us would call him crazy, but legally it could be argued that he's not. A crazy person would act on impulse, not attempt to conceal the crime, and not feel guilty about it. The narrator here defies all these conventions.