  This work documents the initial 3D calculations to simulate the coupling
between radio-frequency (RF) waves and plasma in discharges of the SPIDER
device. Axisymmetric 3D calculations in the plasma domain alone compare well
against equivalent 2D cases. A model of SPIDER driver, the cylindrical chamber
where the plasma is heated by the RF drive, is then defined including the
metallic parts of the Faraday shield, insulator and vacuum layer up to the RF
winding (not included in the calculation domain). Estimates of the power share
in the different parts are obtained using experimental conditions and plasma
data. The results are sensitive to the particular geometry of the driver and
the temperature of the Faraday shield, but generally agree with the
experimental knowledge. The ratio between total delivered power and plasma
absorbed power is found, depending on the plasma parameters, in the range
30--45\%
