As others have said, ambient temp will affect your smoker's ability to hit target temp and would affect the amount of fuel burned during a cook.
I've never seen a 3lb full butt in my life, so I'd wager that what you had was a split/partial butt. Cook time for butts (and any other meat) isn't actually determined by the weight, but rather, the thickness of the piece of meat (at it's shortest dimension). Say that you have a butt that is 12 inches long, 8 inches across and 5 inches thick. If you chop 4 inches off the length, the resulting butt would be 8 inches long, 8 inches across and still 5 inches thick. While the original butt might have been 8lbs and the partial butt only 5lbs, the cook time for both butts would be the same as both are 5 inches at the shortest side.
A way to visualize it is to picture cooking a foot long hotdog that is 1 inch thick. Cutting 6 inches off the end of it won't affect the cook time even though you've cut the overall weight in half.