Laying of underground AC cables
415Vac (f = ??) 3-phase is not a low voltage.

At power frequencies (50 or 60 Hz.) you  can use single wires to make up the cable provided that the insulation is up to standard. This is the cheapest option.

Every country has electrical standards that must be obeyed.

Underground cables  must be carried in a suitable conduit (orange) at a depth of at least 600mm. Before filling in the trench there must be a labeled plastic film covering the length underground - just in case someone tries to dig holes over the cable.