Are you looking for 3" diamond grade 3m striping? I don't think they make it. NFPA regulations state 6" striping if you aren't familiar with NFPA regulations go look them up.
We use a lot of Reflexite (Oracal) striping and the guys here like cutting/installing it a whole lot better than the diamond grade and it has a better reflectivity to it also.
One of our suppliers can also print it so that you don't have to place the alternating stripes.
If they aren't concerned with NFPA regulations then I would recommend doing a print on IJ680-CR, we do a lot of those here also.