The Southampton class was not originally a stock ship, but was kitbashed by Sergbuto. It is very good though, and looks almost exactly like the original.
If you are referring to differences in the silhouettes, the best way to tell them apart is by comparing the funnels and superstructure. You will notice that the Fiji has a somewhat larger superstructure than the Dido, and different funnels. The Southampton has the same hull and superstructure as the Fiji, but has the funnels of the Dido. All three have varying armaments as well.