See answers to your questions inside

1. 有个三联单,是现在要填还是登陆时填? -- The officer will fill them out.

2. cash amount要多少? -- It doesn't matter and they don't care how much cash you are bring with you.

3. fund proof是银行证明吗? -- Yes, a letter from your bank stating your account history and your current balance.

4. 如加拿大海关问是否要quit 美国的工作, 怎么回答? -- Don't worry, they are not going to ask you. If they ask, you can say you are going to quit it once you've found a job in Canada.

5. 美国海关问去加拿大干什么,怎么回答? -- Be honest to answer US border officer's questions. You have to say you just landed in Canada, and they can also see this fact on your passport.

Good luck!!