Sentence 1 is correct: “It is reported that it is going to rain tomorrow.” Here, both the main clause “it is reported” and the relative clause “that it is going to rain tomorrow” are correctly rendered in the present tense. As we know, “going to” is a form of the future tense in English that’s used to make a prediction based on evidence that’s currently available. The grammatical form of the prediction is “going to rain tomorrow” and it correctly uses the present tense “is” as linking verb in the clause “it is going to rain tomorrow.”
Sentence 2 is also correct: “It is reported that it would rain tomorrow.” It uses the modal “would” to indicate the high probability but not the certainty of rain tomorrow. Since weather can’t be predicted with 100% certainty, it’s incorrect to use the modal “will” in that sentence.
Sentence 3 is grammatically incorrect: “It is reported that there is rain tomorrow.” The relative clause “that there is rain tomorrow” is a wrong form of the future tense. When corrected, Sentence 3 will take the form of Sentence 2.
