A triose is a monosaccharide, or simple sugar, containing three carbon atoms. There are only three possible trioses: L-Glyceraldehyde and D-Glyceraldehyde, both aldotrioses because the carbonyl group is at the end of the chain, and dihydroxyacetone, a ketotriose because the carbonyl group is in the middle of the chain.
Trioses are important in cellular respiration. During glycolysis, Fructose-1,6-diphosphate is broken down into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. lactic acid and pyruvic acid are later derived from these molecules.