Player 1 button 3 is at pin N, and player 2 button 3 is at pin 15.
Solder Side | Parts Side
________________________________|___________________________________
-5V | A | 1 | +12
Speaker | B | 2 | Speaker
2P Button 2 | C | 3 | 2P Button 1
2P Left | D | 4 | 2P Right
1P Start | E | 5 | 2P Start
1P Button 1 | F | 6 | 2P Up
1P Button 2 | H | 7 | Service Switch
1P Right | J | 8 | 1P Left
1P Up | K | 9 | 2P Down
Coin (1) | L | 10| Coin (2)
1P Down | M | 11| Coin Counter (1)
1p Button 3 | N | 12| Coin Counter (2)
Video Green | P | 13| Video Blue
Video Red | R | 14| Video Sync
| S | 15| 2P Button 3
GND | T | 16| GND
GND | U | 17| GND
+5 | V | 18| +5
There are 2 boards that use the full pinout, Super Basketball and Strategy-X. I own them both. Strategy-X might be really rare, but Super Basketball is really common.
I already modified my Arcadeshop adaptor to support these titles myself. But I figure if you are gonna do them, then you might as well do them right.