3) I have learned few things but I have studied little time Your first two sentences are fine. I am not sure exactly what you are trying to say with number 3. Perhaps, I have not learned very much but have studied very little?

Sentence one is fine. Sentence two is ok, but you might not want to use the word "little" so close together. Perhaps "I feel a little tired. I have to sleep for a while. Sentence three is understandable, but in the USA we would probably say, "I have learned a few things, although I have studied little", if you learned those things with little study time. If you feel you didn't learn much because of a small amount of studying, then perhaps you would say, "I have learned only a few things because I have studied little".