Three millimeters in width is approximately one-tenth of an inch. A penny is just over 1.5 millimeters, making 3 millimeters about the width of two pennies.
A millimeter is a measurement in the metric system and is equivalent to one-thousandth of a meter. When viewing a meter stick, the millimeter is the smallest measurement displayed. When comparing metric to standard measurement, an inch is equal to 25.4 millimeters. The United States Mint measures coin widths in millimeters. A dollar coin is 2 millimeters wide, a dime is roughly 1 1/3 millimeters wide and a nickel is nearly 2 millimeters wide.