If it's good quality quarter-sawn timber, especially if it's well-figured, then one piece works great. Three piece necks are a good way to reduce the risk of warping as the timber ages, as you can oppose the grains so any warping will be cancelled out by the other piece - handy to make use of otherwise-nice wood that isn't quarter-sawn. Also you can add a contrast veneer between the pieces for a distinctive look.
If I were building a custom bass, I'd aim for a single piece of flamed maple, but if it was a through-neck I'd probably opt for a three piece for looks.