How can I help my mother to come for short visit to US?I am green card holder.Three years ago I applied for her visit visa and rejected

OK mom can apply for a visitor visa, even if the visa application was rejected years ago. Mom must independently qualify on her own, by submitting financial documents to show that she will return upon completion of her visit. Proof of that intent would be a letter from her employer showing steady and stable proof of income, bank statements showing deposits of those earnings from her employment, paystub's showing earnings from her job, tax returns showing income from that employment source. She can additionally submit evidence that she owns property and that she owns equity in that property like a condominium or a house. These documents will convince the US consulate officer that she would most likely return after her visit. She also would be required to show a sufficient amount of income in a bank, that would provide proof that she can cover her expenses while she is visiting the United States. Your participation in the application process for mom's visa is not even really required.