The three-mile limit refers to a traditional and now largely obsolete conception of the international law of the seas which defined a country's territorial waters, for the purposes of trade regulation and exclusivity, as extending as far as the reach of cannons fired from land.
[1] In Mare Clausum (1635) John Selden endeavoured to prove that the sea was in practice virtually as capable of appropriation as terrestrial territory.
A workable formula was found by Cornelius Bynkershoek in his De dominio maris (1702), restricting maritime dominion to the actual distance within which cannon range could effectively protect it.
It has also been suggested that the three-mile limit derived, at least in some cases, from the general application of the league (a common unit of measurement at sea) rather than from the range of cannon.
Commonly these maritime territories extend 12 nautical miles (22 km) from a coastline, and this was eventually established as the international norm by the 1982 United Nations Convention on the Law of the Sea.