The pin configuration and set up of ACC2 for KPA500 or other linear amplifier connection to KX3 is not clear on P.5 of KX3 manual. Correspondingly, P.21 of KPA500 manual do not have specific information about connection to KX3.
I communicated with Elecraft support team who gave the following information which I would like to share with you:
1. The keying of KPA500 is supposed via the ACC2 of KX3. The correct pin configuration of ACC2 for the Key line is Ring and Sleeve.
2. In the KX3 menu, turn ACC2 'off'.
Perhaps, Elecraft would like to update the operation manuals in due course.