It is possible though unlikely that your tank has cycled 3ppm of ammonia in one week. It is more likely if you added some material from a mature tank (e.g. filter sponge, gravel, ornaments).
It it also possible that your ammonia test kit is faulty and giving a false zero. Did you test the level of ammonia immediately after you added it?
Your tank will be cycled when both your ammonia AND nitrite are 0ppm within 24 hours of adding a full dose of 3ppm ammonia. This normally takes several weeks.