The threepence was part of the now defunct pound, shillings and pence system. The British exported this system to various parts of their empire. Often the size, shape, metal and weight of the various versions of these coins was more or less uniform across the Empire: this can be observed in the shillings, florins and halfcrowns. However, the lower the denomination, the more likely the different coins were to show variations. The threepence is a nice example, because most of them did match one another in size, shape, weight and metal, and the different designs are interesting to look at. But there were a few that deviated in various ways from the standard.
The silver and cupro-nickel threepences of the UK and the various countries of the Empire that used it were around 16mm in diameter and around 1.4 grams in weight. I'll start with this UK threepence of 1925, because the reverse design it carries had been more or less standard since 1822, but from 1927 onward a new design was adopted.