Why does Newton's third law exist even in non-inertial reference frames?

Question

While reviewing Newton's laws of motion I came across the statement which says Newton's laws exist only in inertial reference frames except the third one. Why is it like that?

Answer 1

Because, unlike the other two laws, the third law is totally contained between two systems, and only concerns the forces they exert on each other. It is not making any universal statements about net force or overall change in motion. Two systems acting on each other will always exert equal-magnitude, opposite-direction forces on each other, no matter what else is going on.