When you program a new key on slot 3, it will overwrite the stored data stored by BMW (which is known by them when they want to make a key).
Ordering a new key from BMW, it will not work this way, no matter what you do.
We always recommend Autohex II users to add key on Slot 9 then 8,...In this E90 if you don't have any working key, you must work with Autohex as a no working key case (Option 4)
If you need any help to complete this job when you get the blank key, contact support, they will work with you on teamviewer.
tech.support15@micro-tronik.comCheers