triacid

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

• adjective Capable of reacting with three molecules of a monobasic acid. Used of a base.
• adjective Containing three replaceable hydrogen atoms. Used of an acid or acid salt.
• noun An acid containing three replaceable hydrogen atoms.

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

• adjective (Chem.) Capable of neutralizing three molecules of a monobasic acid or the equivalent; having three hydrogen atoms which may be acid radicals; -- said of certain bases; thus, glycerin is a triacid base.

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

• adjective chemistry Capable of combining with three molecules of a monobasic acid