The words threw, through, thru sound the same but have different meanings and spellings. Why do threw, through, thru sound the same even though they are completely different words?
The answer is simple: threw, through, thru are homophones of the English language.
Past tense of throw.
In one side and out the opposite or another side of: went through the tunnel.
Among or between; in the midst of: a walk through the flowers.
By way of: climbed in through the window.
By the means or agency of: bought the antique vase through a dealer.
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.