See this wikipedia article:
http://en.wikipedia.org/wiki/Molecular_Borromean_rings
Chemists took on the challenge of creating a compound with the structure of the famous Borromean rings, three rings linked together among which no two are linked. The compound depends on six zinc ions, represented by the balls in the photo, for its synthesis and stability.
The STL provided is scaled to fit glass marbles, 16mm in diameter. It can be scaled to fit other sized balls, for example the pinball version also pictured.
Echoing the actual compound, this model tends to “lose its marbles” unless all of the marbles are fitted in place.

Printed in place, prelinked. This requires a lot of support. After printing and cleaning up, place the marbles, which are locked into place by slight concavities on the inside edges of the rings’ curves. Make sure the rings are all mutually perpendicular for the balls to stay in place.