Tekken 3 is the only arcade game I have that needs more than 2 buttons. 
I haven't wired up the extension connector (as I don't have one yet) but do have the 3rd button connected via the normal Jamma adapter. The problem - it doesn't do anything! 
I've tried messing about with it for numerous hours as the cabinet was originally Electrocoin but now I've bypassed that and connected the 3rd buttons directly to the Jamma connector - still no joy. 
I've tried connecting these same buttons to service / test switch and the buttons definately work before you suggest that. 
I now have a feeling that both kick buttons come directly off the extension connector - anyone know if this is the case? 
Any help / links would be appreciated (BTW - I've found bugger all in the link department, even Namco don't have the manual on their site...)