1. everyone knows some of easy ones like 1,2,3, a person, a tree. and some know more.
2. if you learned korean words there are lots of hanja. But they would be hangul version of them. meaning is the same.
3. We don't actually use hanja itself. We don't write hanja because we don't have to. occasionally some people whose name is in hanja, have to write their names doing some official stuff relating to real estate document something or in DMV.
1. Yes. It’s recommended in Korea for young students to learn Hanja, because a lot of Korean words consist of Hanja.(Not that Hangul which was invented by King Sejong consist of Hanja, not at all. But in terms of meaning, I mean.) It might be difficult for non-Korean speaking people to grasp, but Hanja contains certain ‘meanings’, so learning Hanja can help you understand much more about words’ meaning.
3. As tummytight said, it’s used in official documents. And it’s also used in newspapers functioning as symbols esp. in titles. Ex) 미국-美. (Instead of writing all the letters) And it can be written inside parenthesis to clarify meanings of rather unused/difficult words or homographs(words that spell the same but have different meanings).