When you get the 3 beeps it means water has leaked into the base tray and activated the flood switch.
Now if you are looking at fixing this yourself then you need to remove the outer wooden door and then remove the wooden plinth at the bottom of your kitchen.
Then make sure its unplugged then you need to remove the metal cover below the door at the bottom.
There you will see the workings. In the base tray you will see water.
Use a sponge and soak all the water up and dry the base.
This will reset the flood switch and allow it to work as normal from there.
But you need to test it and see if somthing is leaking under there as it can be a hose or a motor thats leaking.
But if you dont fancy doing that then you will need to get an engineer in.
The best people to ring is Service Force as they are the repair agents for AEG and carry a few parts for them.
There number is *****(###) ###-####and im sure they can sort it for you from there.