Magnets will not stick to the plastic bumper (front or rear).
Here are the locations to stick them on (large ones on the rear sides, small ones on the front bumper; see
Clean Air Vehicle Decals for Using Carpool and HOV Lanes - California DMV ):
If you are wanting to apply them with magnets so that you can immediately remove them after your drive (which you should to avoid theft), there is no place on the front bumper of the Model 3 that will work. For the rear side stickers, magnets will work on the quarter panel above the bumper seam.