If I was putting a 3 piece crank on a bike that had a one piece crank I would use a sealed bottom bracket. However, many bikes come with 3 piece cranks with an unsealed loose ball american bottom bracket. All the Nitrous, Kink Curb, Stolen Casino, just to name a few. They do use a different spindle which won't work with a sealed BB, but since you get a spindle when you buy a set of cranks, you just need a sealed american bottom bracket and the set of cranks.