I had one of these a while back, so I'm working from memory how to clear this fault, but I think it went like....
1. Remove your ink cartridegs.
2. Remove all the paper.
3. Power down the printer by removing the mains plug.
4. Unplug the printer cable (parallel or USB).
5. Wait at least 5 minutes.
6. Power Up the printer.
7. Wait for initialise sequence to end.
8. If the 3 lights are still flashing, the printers processor is indicating an unrecoverable fault, and probably only Xerox could help you.
9. If just the one light flashing, turn the printer off with the pushbutton, reconnect the signal cable, and turn it back on. After initialisation, re-install ink, and paper.
10. Hopefully all should be OK.