The words tern, terne, turn sound the same but have different meanings and spellings. Why do tern, terne, turn sound the same even though they are completely different words?
The answer is simple: tern, terne, turn are homophones of the English language.
A three-masted schooner.
Any of various sea birds of the genus Sterna and related genera, related to and resembling the gulls but characteristically smaller and having a forked tail.
Games A set of three, especially a combination of three numbers that wins a lottery.
To cause to move around an axis or center; cause to rotate or revolve.
To cause to move around in order to achieve a result, such as opening, closing, tightening, or loosening: turn the key; turn a screw.
To alter or control the functioning of (a mechanical device, for example) by the use of a rotating or similar movement: turned the iron to a hotter setting.
To perform or accomplish by rotating or revolving: turn a somersault.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.