There's literally proof that China occupied Aksai Chin. So there's no arguing to that it belongs to India.

As far as the other two go, Read about the Instrument of Accession signed by Kashmir's king. Kashmir is and was always a part of India. An election was held before the Indian independence to decide whether to divide India & Pakistan. And the election resulted in creation of Pakistan. The land they wanted was given according cutting India in the left and right. (East & West Pakistan). And Pakistan suddenly comes and says, no the land is ours because there are more Muslims and starts occupying land. That doesn't nake it Pakistan's land.