For example:
If my car consumes an average 8.8 litres of gas per 100 kilometers, a 350 kilometer trip will consume 30.8 litres (and therefore $61.6 if gas price is $2/litre)...
...or a fast currency converter: if 1 Euro is about 1.09 US dollars, 33.9 EUR is 36,92 USD.
Three can also be used for measurements, image sizes, grocery shopping and more.
As simple and trivial it is, the rule is used daily in practical arithmetics.
The application has a simple and modern interface to perform fast calculations and includes some basic options to facilitate the displaying results (thousands separator, decimal digits) including color customization in dark/light modes.