Hello,
I currently updated an UM3 to the latest stable firmware (3.6). I am not sure how it behaved in older versions, but I recognized, that it is not possible to change the material flow parameter for only one extruder. So if I change the value for extruder 1 it is automatically change for extruder 2 as well. I used the 'Tune' menu on the printer to change the value.
I don't think that this is the right/desired behavior. The flow might be different for different materials that are used in the same printing job. If it - why ever - is the right behavior I suggest to extract the setting from the 'Extruder 1/2' menu to a separated entry, because it suggests to be changeable per extruder and not in general.
best regards,
ahorn42