This Glc3 trisaccharide [Glucose a(1->2) glucose a(1->3) glucose] consists of the 3 terminal sugar monomers found at the non-reducing end of the Glc3Man9GlcNAc2 oligosaccharide.  This oligosaccharide is a critical factor in the N-glycosylation pathway and in protein folding quality control.  It is co-translationally added to nascent glycoproteins by the oligosaccharyl transfer complex in the endoplasmic reticulum.  

This triglucose molecule is the minimum substrate for the next enzyme in the pathway, Processing Alpha-glucosidase I, which cleaves the glycosidic a(1->2) linkage, releasing glucose.  

The three-dimensional configuration was determined from molecular dyamics simulation of this substrate within the active site of Processing alpha glucosidase I (PDB ID 4J5T).