First thing to do would be to swap player 3 and player 4 display glass to see if the issue moves with the glass or not. If issue moves to player 4 then it is a display glass issue or solder connection, if it stays with player 3 then likely it is on the Alphanumeric Master Display Board or perhaps the connector to display 3. There is not much there but IC's. The segment that is out is segment a and that segment is driven through R92 for both player 3 and 4 so I would not suspect the resistor. Both 3 and 4 displays segments are driven by U9, U10 and U11 so I would not suspect those since player 4 is okay. That leaves the strobes for each display which go through U6, U8 and U7 but the strobes are used for the digit and not just a segment so I would not suspect those either. It would be best to have a logic probe to trouble shoot. My guess is just bad connector from the master display board to the display itself. Look at those closely when you swap the displays. Also check to make sure there are no cracked solder joints on the display board.

I should add that you do the swap with the power off. Also, be careful around the connectors with power on, there is +100 and -100 volts on the display. Segment a is pin 20 on the display board and pin 1 of the glass itself.