When you hit the replay button it should:
- fire the Credit Unit subtract solenoid and
- fire the Coin relay which
- starts the Score Motor which
- advances the Total Play meter and
- advances the Coin Unit

It sounds like all of these are happening for players 1 and 2, but you should probably double check. How far through the list do you get for player 3?

/Mark

If nothing happens when you press the credit button you need to sort out this circuit:
Space Mission credit button (resized).jpg
It could be that one of the switches in the red box is closed when adding player 2 but open when you try to add player 3. It's possible but maybe less likely that either the Reset relay or Score Motor index switch on the left opens after player 2 is added.

If you have a alligator clip test lead, clip it in around the switches one at a time to see if bypassing the switch gets things going.

Hmm. That switch doesn't look right. It's bent over much more than it should be. Did you disassemble that stepper? It looks like the pin should be below the long or traveling switch leaf and the switch should be normally closed. Each time you hit the credit button the Coin Unit advances and the pin moves up until at the last (4th?) step the pin opens the switch which disables the Replay button.