The lower %, the better. I kept my grades to 2.5% and I have no issues even with long consists.
3% isn't too much of an issue for most reasonably sized consists. Very important is the easement going into the grade and coming out of the grade. That will have a major effect on the reliability of running. I used about three feet at the start of the grade and at the top of the grade to taper back to level track.
