A senior police officer said all the three injured were rushed to Medanta hospital in Gurgaon and their condition was very critical.
The officer said the other two are Singh's personal security officers.
The incident took place at Raghunandan Vatika, where they went to attend a private function.
Police stated that the Indian National Lok Dal (INLD) leader Singh suffered bullet injures in his head.
A similar attack took place on Singh in June 2012. Then an MLA, he was shot at by assailants outside his office in a south west Delhi locality. One of his relatives had also received bullet injuries at that time.