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I just checked my notes and I did the exact smoke you are talking about. Three 8lb butts on an 18" WSM.
My time was the same as smoking one butt, but did burn through fuel faster to keep the temps up with more meat in the smoker. You will probably need to add more fuel somewhere between hours 9-10 running at 275. Keep an eye on your pit temps and fuel level and you should be good.