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).
Three thousand has a sound change where せん becomes ぜん. This is called rendaku (sequential voicing). Similarly, 八千 (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.