The game MUST detect all 3 balls before it will start. Run diagnostics, go into switch tests and see if it finds the 3 switches where the balls are resting.

Yup, on that machine all three balls must be closing the outhole trough switches or you can't start a game.

Could be this, and easiest check is a switch test. I had a driverboard go bad (which controls lighting, switches, and drives coils), so it had nothing to do with the playfield. It could be board related. I would start checking components, if you're not up for that and want a quick fix, I HIGHLY recommend replacing it with a Kohaut PCB:
http://www.pinballpcb.com/System3-7DriverBoard.html

It's bulletproof, works great with this title, will make it rock solid, and has nice diagnostic LED strips on it.

The upper playfield ball lock trough is included also, so any combination of balls in the outhole trough & upper lock trough must equal 3 balls.

One easy way to divide the problem is to try to start the game with 2 balls in the upper trough, and 1 in the lower. If that works, the problem is in the lower PF trough.

Also sound like you have a blown solenoid fuse.

Check the connectors at the top right corner of the driver board. There are two 9pin ones that handle switches.

Down targets = switches.... Thats why you had 9.

Sol fuse bad... It is a 2.5A SB on the power board, I forget if it is the 2nd or 3rd from the right. Manual will show.

One enemy does not stop the Black Knight....