Citroen C3 heater not producing hot air.

First thing to check would be coolant level . I f that is o.k a common fault is the air distribution flap which is usually operated by an actuator / motor which often fails . On some earlier cars it was cable operated but I would think yours is motor operated . Another possible cause is a thermostat stuck open which means the engine runs cooler and is reflected in engine temp reading on gauge if fitted .

Think your system is fully electronic with heater damper motors ,These can go out of sync and require calibration to get air flowing correctly,And in some cases motors fail .