I would say only 2 and 3 are correct. In English you can only use certain nationalities as nouns. These are generally those that end in -an (You sound like a Canadian, Brazilian, German etc). For other nationalities you would have to use it as an adjective and add a noun after, such as guy, person etc (You sound like a Chinese person, French Person, Portuguese person etc). Using these nationalities as nouns is a very common mark of a foreign language learner. *EDIT: There seems to be some lively discussion as to the validity of sentence 1, so I just want to qualify that my comments are based on my own experience in my region as a native speaker. For me, "You sound like a Chinese" makes it sound like the person is being offensive or is not a native speaker, however it's entirely possible this is a valid construction in other regions!

All are grammatically correct. The differences are: 1. You sound like a Chinese. The person is not Chinese. 2. You sound like a Chinese guy. Same as 1 but a male Chinese. 3. You sound as if you were Chinese. The person sounds like how most a Chinese would speak. This sentence has more emphasis placed on how the person speaks. A shorter way is "You sound like Chinese"

All three are about the same, but number one is a bit informal. Number two and three are more commonly used. If I had to pick one, number three is probably the best