trimorph

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A substance that occurs in three distinct crystalline forms.
• noun One of the crystalline forms in which a trimorphic substance occurs.

from The Century Dictionary.

• noun In crystallography, a substance which exhibits trimorphism, or one of the three forms which such a substance presents: thus the term may be applied to titanium dioxid, or to rutile, one of its three forms of crystallization.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Crystallog.) A substance which crystallizes in three distinct forms, or which has three distinct physical states; also, any one of these distinct forms. See trimorphism, 1.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun crystallography A substance which crystallizes in three distinct forms, or which has three distinct physical states.
• noun crystallography Any of the three forms of such a substance.