DIAGNOSIS That's the machine's way of telling you it needs a descale. The three flashing lights (usually the coffee, steam, and hot water lights) is the universal "descaling required" signal on these De'Longhis. It's not a breakdown, just maintenance time.
FIX You'll need to run the descaling program. First, get a bottle of De'Longhi-branded descaler or a citric acid-based descaler made for espresso machines. Don't use vinegar. Empty the drip tray and water tank. Mix the descaler with fresh water in the tank as the bottle instructs. Put a large container under both the steam wand and the coffee spout. Turn the machine on, press and hold the steam button for about 5 seconds until all the lights start flashing. The machine will now pump the solution through. It'll take 20-30 minutes. When the tank is empty, refill with fresh water and run the program again to rinse.
IF THAT DIDN'T WORK If the lights keep flashing after a proper descale cycle, the machine might not have registered it. Try unplugging it from the wall for a full five minutes to do a hard reset, then plug it back in. Sometimes the control board just needs a fresh start.
HEADS UP Don't ignore this. If you keep trying to use it with the descale light on, you can eventually get scale blocking the internal thermostat, which is a much pricier fix. Run the descale every couple of months if you have hard water.