Extract from Pinball guides:

Third Flash Completed:
The Fakers Guide: a third flash means the U8 5101 RAM is good. No third flash means U8 (5101) or its socket is bad. Not on Stern M-200 board the third flash is also testing the U13 RAM (5101). Additionally a bad 6800 processor at U9 can also only allow the MPU board to flash twice and stop (no third flash.) I know that sounds strange, but I've seen it happen. So if you know the 5101 is good and there's no battery corrosion, try replacing the 6800 at U9 with a known good version.

You may have to replace the sockets as well as the ram.
U8 is right in the corrosion zone.