If the Wikipedia article is not out of date, it appears you'll need 600 words, that is 300 for level three plus 300 from levels one and two. You could ask a teacher for an accurate list and study 20/day in Anki.
1.you need make phrase about the character ,it helps you remember the meaning fast.
2.put the characters in one sentence.because HSk3exam is about sentence and dialogue.
3.find the test paper of past time.you will know what's the way the test wants to check.
4.Don't worry so much.keep calm to learn.
Good luck !