For clarity's sake: Dick Teague didn't design the AMX-3. He headed AMC's design department, and assigned the project to a team that included Bob Nixon, the designer mostly responsible for the Javelin and the production AMX. The car was first displayed at the Chicago Auto Show in February of 1970.
The Mazda RX 500 was designed in-house at the Toyo Kogyo, and was revealed at the Tokyo Auto Show in October of 1970.
Other than some detailing of the air extractors on the front lids of each, I'm not sure how anyone could confuse one for the other, or suspect one design was copied from the other. But that's just my opinion, of course.