The difference between noimod=2 and noimod=3 in bsim3
is in the channel thermal noise. noimod=3 uses regular SPICE-like
thermal noise [8kt/3*(gm+gds+gmb)]. noimod=2 uses
the inversion charge (Qinv) to calculate the channel thermal noise. I think the SPICE-like channel thermal noise model (noimod=3) was derived for strong-inversion region.
In the deep-subthreshold region, gm, gds, and gmb are extremely small and may under-estimate the noise.
noimod=2 (the bsim3 noise model) is based on the inversion charge, which is more applicable to all operation regions.
The SPICE-like channel thermal noise model is included in bsim3 (and other bsim models) are for backward-compatibility with older MOSFET models. You can consider this is a defficiency of noise model from previous model generation.