Three millimetres is around one-tenth of an inch in width. A penny is somewhat more than 1.5 millimetres in diameter, so 3 millimetres is roughly the width of two pennies.
One millimetre corresponds to one thousandth of a metre in the metric system. The millimetre is the smallest measurement displayed on a metre stick. An inch is equivalent to 25.4 millimetres when comparing metric and standard measurement. The United States Mint uses millimetres to measure coin widths. The width of a dollar coin is 2 millimetres, the width of a dime is around 1 1/3 millimetres, and the width of a nickel is approximately 2 millimetres.