Morning Simon
Welcome to the Forum
I think you may have posted in the wrong spot, if I am reading your post correctly it says X3 not 3 series
However BMW like to place the battery in the boot under the floor, if they are charged or jump started directly at the battery damage can be caused to the electrical system. The car should be charged at the posts under the bonnet or jump started there as well. I hope that is the case with your X3 and that it hasn't been jump started at the battery.
Normally disconnecting the battery negative post first then live (some people touch the 2 terminals together to discharge any static) and leaving it for 30 mins then reconnecting 30 mins is important as the car takes a while to self test and shut down systems. Reconnect Live first then Negative this should reset everything. If not then you will need more help as your car has several computers that need to see/communicate with each other. May be a member near you who has a Diagnostic program like Inpa/Insta or PA Soft 1.4.0 or a reader capable of fault reading a BMW can help?
Fingers crossed it is simple
Dave