1-acyl-2-(9Z,12Z)-octadecadienoyl-sn-glycero-3-phosphocholine + 2 Fe(II)-[cytochrome b5] + O2 + 2 H(+) = 1-acyl-2-(9Z,12Z,15Z-octadecatrienoyl)-sn-glycero-3-phosphocholine + 2 Fe(III)-[cytochrome b5] + 2 H2O — reaction