Explanation:

Fringing reefs are coral platforms lying close to the shore and extending outwards from the mainland and separated from it by a shallow lagoon. The outer edge of coral grows rapidly because of splashing waves that continuously renew the supply of nutrients and salt water.
The Barrier reef is situated further away from mainland and separated from it by much wider and deeper lagoon or channel. The reef is partially submerged and there are small openings that allow for water flow across the reef and some navigation also.
(Generally reefs pose a problem to navigation)
 Atolls are similar to barrier reefs except they are circular in shape enclosing a shallow lagoon without any land in the centre. The encircling ring is broken in few places to allow the free flow of water. These reefs are useful for fishing.