To visualize 3 centimeters, think about the end of the average thumb. From the first joint to the tip is approximately 1 inch. Three centimeters is approximately 1 3/16 inches, so 3 centimeters is just slightly longer than the average thumb from the first knuckle to the tip.
Each inch equals 2.54 centimeters. Three centimeters is about half a centimeter longer than 1 inch. On a standard ruler, find 1 inch and add 3/16 of an inch for an estimate of 3 centimeters. Other visual representations of approximately 3 centimeters include a dollar bill folded in half lengthwise, for a measure of about 3.2 centimeters, or a the diameter of a quarter, which is 2.5 centimeters across.