那么, 如此 and 太, these words are functional as adverb, it modify the word 难. Your chinese writting in these three sentences are grammatically correct and understandable.
Which is better? It depends on you, the wirtter, I mean the person who put it into application. In English, it is called the writting style. Many people who claimed themself Native Chinese speakers or scholars with their attempt to confuse the learners by giving lots of definitions. In my opinion, all sentences mean the same, it expressed the idea of leaning Chinese is very difficult.