The words bel, bell, belle sound the same but have different meanings and spellings. Why do bel, bell, belle sound the same even though they are completely different words?
The answer is simple: bel, bell, belle are homophones of the English language.
A hollow metal musical instrument, usually cup-shaped with a flared opening, that emits a metallic tone when struck.
Something resembling such an instrument in shape or sound, as:
The round, flared opening of a wind instrument at the opposite end from the mouthpiece.
A percussion instrument consisting of metal tubes or bars that emit tones when struck.
A popular, attractive girl or woman, especially the most attractive one of a group: the belle of the ball.
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.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").