Voltage Transformer Question

Both 5 legged and 3 legged cores are "core form." While the 5 legged core has a few characteristics of a shell form transformer, it does not have all of them. Shell form transformers and 5 legged core do have similar characteristics for 3rd harmonics. Three phase faults are not all that different on any transformer but the 3 legged core is quite different on single phase faults which can actually be an advantage at times. 5 legged and shell are more tolerate of unbalanced loads better but a tertiary winding will help a 3 legged core similarly.