Galactinol synthase involved in the biosynthesis of raffinose family oligosaccharides (RFOs) that function as osmoprotectants. May promote plant stress tolerance (By similarity). myo-inositol + UDP-alpha-D-galactose = alpha-D-galactosyl-(1->3)-1D-myo-inositol + H(+) + UDP Induced by cold in a DREB1A-dependent manner; this induction is accompanied by a reduction in trimethylation of 'Lys-27' of histone H3 (H3K27me3) in GOLS3 promoter (PubMed:19500304). Induced by methylviologen (MV), a superoxide radical generating drug. Belongs to the glycosyltransferase 8 family. Galactosyltransferase subfamily.