How warm can 3 gallons of fermenting ale in a carboy in a tub of water get?

You will get a slightly higher temperature from the metabolic activity of the yeast. I suggest that you take advantage of that tub of water by draping cloth over the carboy and leaving the ends in the water. That way, evaporation will cool the carboy and wort. Otherwise, there is no point in having the carboy in water. A lot of people use old T-shirts as the evaporative coolers, as they are about the right size and shape.