They're all good. The third one should be, "I think it is going to be
raining soon.
Be sure to capitalize the pronoun "I," and the first word in every sentence. Also you need punctuation at the end of each sentence - a period in this case.
All of your sentences are "comma splices." They should be two separate sentences, or should be joined by appropriate conjunctions, such as, "It is cloudy, and therefore I think it will rain."
You have a typo in the first sentence. Use either "It's" or "It is."