Dimorphous means that two minerals have the same basic chemistry, but have different crystal structures. And of course, Trimorphous involves three minerals and Polymorphous is when four or more minerals have the same basic chemistry but have different structures.

A good example of trimorphism is AndalusiteKyanite and Sillimanite. All three have the same basic chemistry, Al2SiO5, but Andalusite is Orthorhombic, Kyanite is Triclinic and Sillimanite is Orthorhombic.

Dimorphous Minerals

Anorthite (triclinic) :: Svyatoslavite (orthorhombic)

Boracite (orthorhombic) :: Trembathite (trigonal)

Catapleiite  (monoclinic) :: Gaidonnayite (orthorhombic)

Cavansite (orthorhombic) :: Pentagonite (orthorhombic)

Cordierite (orthorhombic) :: Indialite (hexagonal)

Cubanite (orthorhombic) :: Isocubanite (isometric)

Natrolite (orthorhombic) :: Gonnardite (orthorhombic)

Phosphosiderite (monoclinic) :: Strengite (orthorhombic)

Pyroxmangite (triclinic) :: Rhodonite (triclinic)

Kurnakovite (monoclinic) :: Inderite (triclinic)

Lepidocrocite (orthorhombic) :: Goethite (orthorhombic)

Zircon (tetragonal) :: Reidite (tetragonal)

Zoisite (orthorhombic) :: Clinozoisite (monoclinic)

Trimorphous and Polymorphous Minerals

Anatase (tetragonal) :: Brookite (orthorhombic) :: Rutile (tetragonal)

Andalusite (orthorhombic) :: Kyanite (triclinic) :: Sillimanite (orthorhombic)

Aragonite (orthorhombic) :: Calcite (trigonal) :: Vaterite (hexagonal)

Forsterite (orthorhombic) :: Ringwoodite (isometric) :: Wadsleyite (orthorhombic)

Diamond (isometric) :: Chaoite (hexagonal) :: Graphite (hexagonal) :: Lonsdaleite (hexagonal)