As I knew

请问加拿大公民申请必须在加拿大住满三年后才能申请吗?
Yes

是不是递交申请后就可以马上离开直到宣誓再回来?
Not exactly., because you need take a paper test in the term

期间还有什么需要办理的吗?
Receive a book about Canadian history, government, etc., then wait for paper test notice, attend the test and pass the test, get new citizen ceremony notice.