If I remember right mg/l is the same as ppm. So if you have 3 mg/l nitrIte then you have 3 ppm nitrItes, which is high. Do a water change to get the nitrIte down to under .25 mg/l and retest. This should also drop your Ammonia and NitrAte, unless your tap water has them in there to begin with. Remember to match your tank temperature when adding water, and dechlorinate it. Your cycle appears to be coming close to finishing. Water changes will not hurt your cycle.