Catalyzes the first and rate-limiting reaction of the four reactions that constitute the long-chain fatty acids elongation cycle. This endoplasmic reticulum-bound enzymatic process allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids (VLCFAs) per cycle. Condensing enzyme that exhibits activity toward saturated and unsaturated acyl-CoA substrates with higher activity toward C18 acyl-CoAs, especially C18:0 acyl-CoAs. May participate in the production of saturated and monounsaturated VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA eicosanoyl-CoA + H(+) + malonyl-CoA = 3-oxodocosanoyl-CoA + CO2 + CoA H(+) + hexadecanoyl-CoA + malonyl-CoA = 3-oxooctadecanoyl-CoA + CO2 + CoA H(+) + malonyl-CoA + octadecanoyl-CoA = 3-oxoeicosanoyl-CoA + CO2 + CoA (9Z)-octadecenoyl-CoA + H(+) + malonyl-CoA = (11Z)-3-oxoicosenoyl-CoA + CO2 + CoA (9Z,12Z)-octadecadienoyl-CoA + H(+) + malonyl-CoA = (11Z,14Z)-3-oxoicosa-11,14-dienoyl-CoA + CO2 + CoA (9Z,12Z,15Z)-octadecatrienoyl-CoA + H(+) + malonyl-CoA = (11Z,14Z,17Z)-3-oxoeicosatrienoyl-CoA + CO2 + CoA docosanoyl-CoA + H(+) + malonyl-CoA = 3-oxotetracosanoyl-CoA + CO2 + CoA H(+) + malonyl-CoA + tetradecanoyl-CoA = 3-oxohexadecanoyl-CoA + CO2 + CoA Lipid metabolism; polyunsaturated fatty acid biosynthesis. Testis. The C-terminal di-lysine motif may confer endoplasmic reticulum localization. N-Glycosylated. Belongs to the ELO family. ELOVL3 subfamily.