What is a 3-piece golf ball?

A 3-piece golf ball has three parts: a rubber core, one firm mantle layer around it, and a cover.

The mantle is the reason the construction exists. On a driver, the club compresses the whole ball and the firm layer keeps spin down. On a wedge, the impact forces concentrate in the outer layers, and the soft cover pressed against the firmer layer under it is what produces greenside spin. Titleist describes the job of that middle layer on its casing-layer page.

Most tour balls are 3-piece: the Pro V1, Srixon Z-Star, Bridgestone Tour B X, and Callaway Chrome Soft. So are some cheaper balls with an ionomer cover, like the Callaway ERC Soft. Three layers do not guarantee a urethane cover or tour-level spin; the cover material still decides that.

A 3-piece ball is not automatically firm. Soft urethane 3-piece balls run from the low 70s in compression upward.