[AtlUmpSteve]

You would NOT walk as U3 with a runner on second; while you aren't responsible for a runner on your base, you have one coming to you, so you set. With 1st and 2nd, you also have runner leaving early responsibility at 1st; another reason to set.

Unless it has changed recently, it has been considered optional for U1 to walk or to set with a single runner at 3rd. That is the ONLY time you would possibly walk except for bases empty. It is my observation that, the option stated aside, that most UIC's prefer that both base umpires set in that case, and every other (as required) where there are any baserunners; and that both base umpires walk with bases empty.