Omega-3 fatty acid desaturase that recognizes a range of 18- and 20-carbon omega-6 substrates. Introduces a double bond in the fatty acid chain three carbons away from terminal methyl group to biosynthesize n-3 (omega-3) polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes). Acts on a number of substrates like linoleoyl-CoA ((9Z,12Z)-octadecadienoyl-CoA, 18:2n-6), dihomo-gamma-linolenoyl-CoA ((8Z,11Z,14Z)-eicosatrienoyl-CoA, 20:3n-6), and arachidonoyl-CoA ((5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA, 20:4n-6), to generate alpha-linolenoyl-CoA ((9Z,12Z,15Z)-octadecatrienoyl-CoA, 18:3n-3), (8Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA (20:4n-3) and (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-CoA (20:5n-3) respectively. Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates.