I think SpectreRF should accept MOS model level 3. The only thing you need to change header of the models according to Spectre syntax. Read Spectre reference to learn about this syntax. But level 3 is not very accurate model. Try to find bsim3 model or if you design RF bsim4 model.
If you want to use spectre that means you have spectre simulator, so you have the documentation.
If no use Google to find that. For example: **broken link removed**