triprismo-

https://doi.org/10.1351/goldbook.T06508
An affix used in names to denote six atoms bound into a triangular prism.
Sources:
Blue Book, p. 465 (http://www.acdlabs.com/iupac/nomenclature/)
Red Book, 3rd ed., p. 245 (https://doi.org/10.1515/pac-2014-0718)