I have tried many times,If you want to exprot BMP ,create an export operation below any texture,give it an unique name,
then select file export bmp,then the software seems does not work!
I can not find the bmp in the export folder.
Then I set the image to 2048*2048,to save a large bmp,the software needs time.
But the werkkzeug3 seems does not confirm my perform,it show ok immediately.
So I think the software needs refresh,and confirm you perform.
Then I select file>save as then exit werkkzeug3,
After that double click the file you saved,after it is opened,
You can select file>export BMP,and you will succeed.
Problem solved.