Virrad, in the SPICE model, is a voltage, but actually it represents numerically the value of the Irradiance.
The irradiance is measured in W/m^2 and considers the contributions of all wavelengths to the power that affects a surface.
As a first approximation, an ideal solar cell can be modelled in SPICE by a voltage controlled current source, that receives an Irradiance (represented by Virrad) and provides a short circuit current proportional to the irradiance, and a diode that represents the open circuit voltage. (subcircuit cell_1).
cell_3.cir refers to cell_2 subcircuit that is a generalized SPICE model of a solar cell that includes series and shunt resistance and two diodes that model other important electrical characteristics of the solar cell.
If you want add a third series solar cell you can do for example:
xcell1 45 43 42
xcell2 47 45 44
xcell3 0 47 46
then you add virrad3 between 0 and 46.