Catalyzes the addition of sialic acid in alpha 2,8-linkage to the sialic acid moiety of the ganglioside GM3 to form ganglioside GD3; gangliosides are a subfamily of complex glycosphinglolipds that contain one or more residues of sialic acid (By similarity). Glycosphingolipids are required for convergence extension movements during early development (By similarity). Can catalyze the addition of a second alpha-2,8- sialic acid to GD3 to form GT3 (By similarity). Can use GM1b, GD1a and GT1b as acceptor substrates to synthesize GD1c, GT1a and GQ1b respectively (By similarity). an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl derivative + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->8)-N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl derivative + CMP + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM3 (d18:1(4E)) = CMP + ganglioside GD3 (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD3 (d18:1(4E)) = CMP + ganglioside GT3 (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD1a (d18:1(4E)) = CMP + ganglioside GT1a (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GT1b (d18:1(4E)) = CMP + ganglioside GQ1b (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM1b (d18:1(4E)) = CMP + ganglioside GD1c (d18:1(4E)) + H(+) Protein modification; protein glycosylation. Lipid metabolism; sphingolipid metabolism. Belongs to the glycosyltransferase 29 family.