(さんぜん)

さんぜん
number
three thousand
1. three thousand
The number three thousand. Note the sound change: さん + せん becomes さんぜん (the せ becomes voiced to ぜ).
三千(さんぜん)(えん)です
It's three thousand yen.
This watch was three thousand yen.
There are three thousand students.
Mt. Fuji is over three thousand meters tall.
This town has three thousand years of history. It's a very old town.

Three thousand has a sound change where せん becomes ぜん. This is called rendaku (sequential voicing). Similarly, 八千(はっせん) (8000) has a different change where the reading becomes はっせん. Other thousands (2000, 4000, 5000, 6000, 7000, 9000) keep the regular せん sound.