A key community panel on Monday rejected plans for a casino next to the United Nations, marking the third casino plan to fail in Manhattan and ensuring no casino will be built in the borough in the immediate future. The rejection of the Freedom Plaza proposal, by a 4-2 vote, came despite last-minute sweeteners by the developer, who promised all of the housing created on site would be affordable. Separate community advisory committees last week rejected proposed casinos in Times Square and Hudson Yards.