Recently, the glutathione S-transferase A3-3 (GST A3-3) homodimeric enzyme was identified as the most efficient enzyme that catalyzes isomerization of the precursors of testosterone, estradiol, and progesterone in the gonads of humans and horses. However, the presence of GST A3-3 orthologs with equally high ketosteroid isomerase activity has not been verified in other mammalian species, even though pig and cattle homologs have been cloned and studied. Identifying GSTA3 genes is a challenge because of multiple GSTA gene duplications (12 in the human genome), so few genomes have a corresponding GSTA3 gene annotated. To improve our understanding of GSTA3 gene products and their functions across diverse mammalian species, we cloned homologs of the horse and human GSTA3 mRNAs from the testes of a dog, goat, and gray short-tailed opossum, with those current genomes lacking GSTA3 gene annotations. The resultant novel GSTA3 mRNA and inferred protein sequences had a high level of conservation with human GSTA3 mRNA and protein sequences (≥ 70% and ≥ 64% identities, respectively). Sequence conservation was also apparent for the 13 residues of the “H-site” in the 222 amino acid GSTA3 protein that is known to interact with the steroid substrates. Modeling predicted that the dog GSTA3-3 is a more active ketosteroid isomerase than the goat or opossum enzymes. Our results help us understand the active sites of mammalian GST A3-3 enzymes, and their inhibitors may be useful for reducing steroidogenesis for medical purposes, such as fertility control or treatment of steroid-dependent diseases.