Certain exactly computable partition functions for a large class of three-dimensional supersymmetric gauge theories can be expressed in terms of a fundamental set of holomorphic building blocks. This can be understood by means of a three-dimensional analogue of topological/anti-topological fusion. I will describe the characterization of these blocks as well as some of their surprising properties. Time permitting, I will comment on the relationship with topological strings and analytically continued Chern-Simons gauge theory.