Smoking 3lb Pork Butt
It is difficult to predict when you will reach your foiling target, but the general rule of thumb for pulled pork at your stated cooking temperature is about 1.5 hours per pound, with some general allowances for margin of error.

I would plan on the entire cook taking about 5-6 hours, but be ready for it to be finished in 4. At about 3 hours, start checking temperatures to determine when you will want to foil your roast.

Final note: If your intent is to make pulled pork, do NOT assume that it is done just because it is at 190f. Your thermometer probe should slide in and out of the meat with virtually zero resistance.