Only 6 countries in the world currently use banknotes or coins with a face value of 3! One of them, the Cook Islands dollar is even more unique than the rest. Although the currency was used only for a few years at the end of the 80s – the beginning of the 90s (preceded and succeeded by the New Zealand dollar), the 3$ banknote survived. It is the only Cook Islands dollar banknote or coin that still maintains the status of legal tender. And of course, because of how special it is, the banknote is also a sought-after collector’s item. Thanks to the redditor u/Show_Green for bringing this to my attention!
Over the years many countries experimented with 3-unit coins or banknotes. For instance, in the 90s, Ukraine had a 3 karbovanets banknote and Uzbekistan had a 3 som banknote. Many examples can also be found in the 19th century. Even the US minted a 3-cent coin in the 1850s! You can read up more on this topic in this excellent article by Peter Symes.