Local structural changes from liquid to amorphous state in three Cu45Zr55-xAlx (x=3, 7, 12) ternary metallic glasses have been investigated by the ab initio molecular dynamics simulation. The atomic structure of the glasses has been analyzed by means of bond-type index method in Honeycutt-Andersen and Voronoi tessellation method. Al-centered icosahedral clusters are identified as the basic local structural units and these Al-centered stable clusters play a key role in the structural heterogeneity and glass-forming ability of the Cu-Zr-Al bulk metallic glasses.