Why is B03 provided as multiple band raster and not as signle grey?

I am guessing that you are using default configuration, which is set for visualization.
To be able to control this flow fully, I suggest you create a new layer based on these instructions.

In the layer’s “Data processing” configuration write:
return [B03];
This should result in one-band layer returning values based on your own configuration.

That being said, we need to look in the template configuration, whether it makes sense that there are 3-band layers or not. But for “production-use” we certainly recommend using your own configurations.