Mine does not have original Gottlieb boards in it so I have no experience with those. Three balls in the game is correct. When you turn it on, does it take about 5 seconds before the displays come on? Are any of the playfield lights cycling and is the ball gate on the lower right flipping open and closed? If all that is happening, you're probably in attract mode. If not, you've got something going on preventing that.

The 510 000 on the displays is probably the current high score. Do you see that High Score light cycling on the backglass as the displays show that value?

One thing you will need to do ASAP is get that battery off the CPU board. Also check that area for any signs of corrosion. Ground mods will be helpful.

One thing that might prevent a game from starting is if the game does not see all 3 balls. You might check the switches associated with the outhole and trough to make sure they look good. You also want to check all the tilt switches and assemblies.

You should be able to get manuals from Pinball Resource if you don't have them.

OK, so with the 5 second delay before the displays come on it would sound like the MPU is booting to me. The other stuff suggests a bad driver board or something associated with it. On mine, once it's in attract mode, that gate should swing back and forth constantly. I think the culprit being the driver board is supported by the fact that the machine speaks at times, which to me suggests the MPU thinks it's in attract mode but nothing else is being driven. I'd check the connectors to the driver board, voltage at the driver board etc. with a close eye on that area around the old battery.

Is the ground strap from the cabinet hooked to the head? The ground mods may also now be a good thing to do.