Necessary like clean drinking water? No. Is it required by law? In Georgia, yes:

40-6-296.
(a) Every bicycle when in use at nighttime shall be equipped with a light on the front which shall emit a white light visible from a distance of 300 feet to the front and with a red reflector on the rear of a type approved by the Department of Public Safety which shall be visible from a distance of 300 feet to the rear when directly in front of lawful upper beams of headlights on a motor vehicle. A light emitting a red light visible from a distance of 300 feet to the rear may be used in addition to the red reflector.

I don't know if 3" is the minimum required by DPS, but you can find some excellent ones at your local auto parts store or WallyWorld...