Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the third alpha-1,2-mannose to Man2-GlcN-acyl-PI during GPI precursor assembly (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Belongs to the glycosyltransferase 22 family. PIGB subfamily.