3 inches to the nearest inch and 3 inches to the nearest half inch both refer to a length not less than 2.5 inches and less than 3.5 inches. 3 inches to the nearest quarter inch refers to a length not less than 2.75 inches and less than 3.25 inches.
This will depend on how cold it is, but on average 10 inches of snow = 1 inch of rain, so 0.15 inches of rain = 1.5 inches of snow. It could be less than in inch of wet snow, or more than 2 inches of powder, however.
18.55 mm is less than one inch--about 0.73 of an inch. Since we usually divide an inch into halves, fourths, eights, and sixteenths, this distance is a little more than 11/16 and a little less than 3/4.
A centimetre is smaller than an inch - one inch is the same as 2.54 cm. When converting from inches to centimetres it will require more centimetres to cover the same distance, thus the number of centimetres will be greater than the number of inches.