Being a sprinter I'd say halodrol or superdrol, as they're pretty dry. Halodrol doesn't give good gains until the 3rd week in most cases, so that wouldn't be the best choice, and superdrol will more than likely have you shutdown pretty bad after 3 weeks, so you would need a conventional PCT to get you back on your feet running IMO. Why 3on/3off? Orastan-A could be a possibility for what you're looking for. Or just use methyl mass (containing propadrol) in your shakes for 3 weeks then normal shakes for 3 weeks. Or you could obviously just use propadrol. I could post a killer stack for sprinter training, but it would have to be longer than 3 weeks.