KaRo, Thank you very much!
gmic 0.bmp 1.bmp 2.bmp -to_gray -a c
and it worked!
From the documentation i read
-a is same as append.
-a c would then mean append along the 'c' or spectrum axis
'spectrum', the number of image channels (size along the 'c'-axis).
(the spectrum is respectively equal to 3 and 4 for usual RGB and RGBA color images).
I did play a bit around using the -print command at various position to see the number of bands vary. Without the
-to_gray-command the final image really does have 9 bands on the spectrum axis and each 8bit monochrome image is read as 3 band color image.