Three-part moulds are used in ceramics to reproduce complex forms that cannot be captured effectively with just one or two parts due to undercuts or intricate shapes. 

Three-part moulds are especially useful for:

• Spherical or rounded forms (like closed pots or figurines)

• Asymmetrical designs with undercuts

• Handles or protruding elements that would lock the form into a simpler mould

Slip casting is a ceramic technique used to create precise, consistent shapes by pouring liquid clay (called slip) into plaster moulds. It's especially popular for making detailed or delicate forms that are hard to shape by hand or on the wheel.