It's been my experience that smaller cuts take longer per# than bigger cuts. I've never cooked butts that small so I'm just guessing here.
Your pit temp can change you time allowance quite a bit, but unless you're cooking very low (200) I'd say 2 hrs/pound for a 3# should come close. Butts can hang at a given temp for quite a while so an extra hour or two might be a good idea. If they're done early, just wrap in foil and place in a small cooler (used as a warmer in this case). The time in the cooler will only make them more tender.
No need to add much if any extra time for the second roast when they're that small.