Hi Dave,
Welcome to pinside!
If the Last Ball (XB) relay on the control bank never trips during a 3 ball 1 player game, Inspect and diagnose this circuit with Alligator clip jumper wires, especially the indicated switch on the 2nd Player (PB) relay.
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features