Can function as a Delta(6) fatty acid desaturase. Introduces a double bond in the fatty acid chain 6 carbons away from carboxy terminal to biosynthesize polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes). Acts on a variety of substrates such as linoleoyl-CoA ((9Z,12Z)-octadecadienoyl-CoA, C18:2n-6) and alpha-linolenoyl-CoA ((9Z,12Z,15Z)-octadecatrienoyl-CoA, C18:3n-3) to produce gamma-linolenoyl-CoA ((6Z,9Z,12Z)-octadecatrienoyl-CoA, C18:3n-6) and (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA (18:4n-3) respectively (PubMed:9480865, PubMed:11972048). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (By similarity). Plays a role in synaptic vesicle recycling by regulating synaptojanin unc-26 localization at synapses (PubMed:18094048). (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z)-octadecatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z,12Z,15Z)-octadecatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. The fat-3 mutants lack detectable Delta6-unsaturated fatty acids and have decreased C20 fatty acids levels, these animals require an additional day of development before commencing egg laying (PubMed:11972048). Impaired synaptic vesicle (SV) recycling characterized by the presence of enlarged SV and the formation of abnormal endocytic membrane-bound structures at synapses. The number of SVs is reduced by 37 percent although SV production in neuron cell body and their transport to release sites are normal (PubMed:18094048). HPO-19 and T05H4.4 are cytochrome b5 reductases required for PUFA desaturation in Caenorhabditis elegans. HPO-19 and T05H4.4 knockdown or mutation alter FAT-3 desaturase activity. Belongs to the fatty acid desaturase type 1 family.