The 3rd wedding anniversary is traditionally named leather anniversary. Modernly, it may be replaced by crystal (or pearls, glass).
The number of colors of the regular traffic lights: green, red and yellow.
The number of rings in most popular paper binders: 3-ring binders.
The largest number of double letters in the U.S. state names: Mississippi and Tennessee.
The number of words in the Utah state capital name, the most and only such U.S. state capital name: Salt Lake City; even more specially, each word has 4 letters.
The most popular number of wheels of an automobile is 4. The second most popular number is 3.
The number of wheels of a tricycle or a “cyclo”.
“3-way STOP” traffic sign.
The minimum number of people on a carpool vehicle or a HOV (high occupancy vehicle) is 2 or 3.
The 3 o’clock direction is right-hand side.
Time is divided into 3 periods: past, present and future.
The (old) popular 3.5” photo print is of the size 3.5”×5”.
The length of a standard “golf” wood-pencil is 3.5 inches (usually without eraser).
TV Screen Aspect ratios: Regular [4:3] (for “Pan & Scan” or “Letterbox”) and “Widescreen” [16:9].
The number of main compass direction names used in the U.S. state names: North, South (for both Carolina and Dakota) and West (West Virginia). Only East is not used.
U.S. state of Delaware’s numbers 1, 2 and 3 single-digit license plates are reserved for the Governor, Lieutenant Governor and the Secretary of State.
Ancient Chinese astrologers divided the night sky into 31 regions, namely “Three Enclosures” (Tam Viên) and “Twenty-Eight Mansions” (Nhị Thập Bát Tú). These mansions are grouped by four 7-mansion symbols for four directions: Azure Dragon of the East, Black Tortoise of the North, White Tiger of the West and Vermillion Bird of the South.
Morse code for number 3 is 3 dots and 2 dashes [ . . . _ _ ]
Flag semaphore signal for number 3 is that of letter C.