Gottlieb system 3 boards are notoriously hard to work on. The boards use a lot of decoder and latch chips to move data around. So pay special attention to the clock signals.

The junk you see on your display is a symptom of the cpu board not communicating with the display board. The ribbon cables are also known to fail and I would swap the out first.

Upon a successful boot you should hear two tones from the sound board and the leds on the cpu board and dmd driver will start to blink.
Good luck!

Sounds like you read through Clays guide on pinrepair.com. The machine will boot with just the cpu board and display board linked up. So don’t worry about the sound cards for now.

If any prior work was done, I’d go back and check that pretty carefully.