I don't know about 3 runs, but I saw on another site a few weeks ago that favorites that win do so by more than 1 run 70% of the time. Not saying that favorites win 70% of the time, but when they do win they also cover the run line 70% or more and that was consistent from -120 favorites to -200 or more favorites. I would guess that winning by more than 2 runs would drop to the 40% range. This doesn't take into account how much the underdogs win by. I would say games decided by more than 3 runs either way would be in the 15-20% range, but just a guess.
