There's various options on the C3 - microphone pattern, Attenuation Pad. Check some basics:
make sure that the attenuation pad is off
make sure you are using the mic in "super-cardioid" or "omnidirectional" mode
make sure you are speaking into the correct part of the capsule.
Perform an elimination test - check the microphone with another cable and ...
Set yourself up a master group channel in Cubase
Route everything into this before routing that to your output devices channel
The master group channel now becomes the max level output of your mix, and the output device channel can be used to control actual volume
Standard practice, whatever type of audio interface you have