Occurring at intervals of three: trimonthly. (prefix)
Occurring three times during: triweekly. (prefix)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "tri-" in a sentence
"I think it's extremely important that whatever follows the MDGs moves beyond a list of non-binding, broad objectives, and that any targets agreed are better linked to binding international bi- tri- and multilateral agreements."
"The aroma of asafoetida is due to a complex mixture of sulfur compounds, a dozen identical with volatiles in the onion family, and a number of less common di-, tri-, and tetrasulfides."