Should I send my AP scores from highschool to a graduate school I am applying to?
I doubt that it would have any effect at all. You might be better to stress other, more recent things. Presumably your transcripts show proficiency in statistics sufficient for entry. 

I doubt it would do harm other than to have someone, possibly, question why you did that. And a 3 isn't a huge benefit, as your undergrad experience showed. 

However, if that is your only CV possibility pointing to knowing statistics, it would be a different matter.