Nuclear Fusion Puzzle

Three puzzle pieces made from (truncated) rhombic dodecahedrons assemble into a cube-like shape. Assembly is confusing and the three pieces mutually block one another. In fact, assembly is impossible with rigid pieces.
The name derives from the fact that these three pieces just don't want to go together. Like the atomic nucleus, once assembly is accomplished the pieces are locked tightly together. Getting them apart is actually not so easy either. This puzzle will test the limits of "strong, white and flexible". Warning: some force is required for assembly.