Hard Reset?
I do not know of any key sequence but a hard reset will sometimes **** off a bunch of error messages for a little while.
If your scanner does not erase the codes and or turn off the MIL light, you might try what is called a hard reset.
In brief, to start the hard reset process, open the hood but close all doors and let your 3 go to sleep - no radio display etc and no key in the ignition.
(I find that it is often a good idea to have the drivers door window rolled down and the key in my pocket whenever fooling about the vehicle.)
Disconnect the ground battery terminal from the main starting battery.
Disconnect the positive battery terminal from the main starting battery.
(I assume that you only have one battery in your 3. If not, disconnect the others as well and leave them disconnected until after all is done.)
Connect the negative cable to the positive cable. (NOT the battery.) You will need a short length, (a foot or so), of light gauge, (14 to 18 gauge AWG, stranded copper), wire to do this as there is not enough slack in the positive and negative battery cables for them to touch each other.
Hold them together for about a minute or so, (some say 5 to 10 minutes); you are discharging memory modules within the engine computer and elsewhere.
Reattach positive terminal to the main starting battery.
Reattach negative terminal to the main starting battery.
Start engine and hopefully no warning lights.
Reset time on the radio - note that the station presets are still there as not everything is erased. Nothing much else needs resetting either.
You can now connect up the other batteries if you have a multiple battery setup.
This is a link to a thread in disco3 re hard reset.
http://www.**************/forum/topic19547.html
Replace the **** with disco3 co uk and put a period in each of the two spaces.