Combine the Campari, sweet vermouth and gin in a large nonreactive saucepan. Add the sugar and gently warm the liquid over low heat, stirring frequently. When the sugar dissolves, remove from heat. Divide the orange slices between 2 large (gallon size) resealable bags or place in a large glass bowl and pour over the Negroni mixture. Seal the bags (removing any excess air) or cover the bowl, and refrigerate for at least 4 hours, up to overnight. Drain the oranges (save the infused Negroni mixture for cocktails), and fit each slice with a Popsicle stick. Freeze the slices on a parchment-lines baking sheet until firm, 3 to 4 hours. Wrap the popsicles in plastic wrap and freeze if not serving immediately. The frozen popsicles will keep about 1 to 2 weeks. Makes 9 popsicles.