Is the fuse correct on my 3-phase circuit?
3mm2 is a very unusual wire size for that location.
Since many years the standard sizes are e.g. 1.5, 2.5, 4.0, 6.0, 10.0 mm2. Maybe the size is 4mm2?
But even for 4mm2, the maximal current would be 40A, but only if the cable is running in free air and only for 2 loaded wires, certainly not within a pipe and/or insulation and certainly not for a 3 phase cable (= maximal 3
wires under full load).
A pipe and insulation will reduce the maximal allowed current to 20 to 30 ampere, dependent on neighbouring cables/wires and the dimensions of the pipe and distance to the insulation.
--&gt; Yes, this fuse/cable installation combination is not suited to protect against fire hazards or cable damages, it is not in line with the EU standards.
The Neutral wire is never loaded with more current than a single Phase. Exactly that is one of the important advantages of the 3 Phase power system. If only 1 Phase and Neutral is used, i.e. 230V, obviously both are loaded with 40A in this example of maximal unsymmetry. If all 3 Phases are used in a symmetric load of 3 x 40A, f.e. for a motor in a star configuration, the Neutral current is zero, since all 3 phase currents add to zero current. The Neutral can not be loaded with &gt;40A current.
ABB has a good overview of wire installations and the maximal load.