I am assuming you want to make a balloon centerpiece with a Mickey Mouse themed foil balloon and two 11" latex balloons, right? The lifting capacity of an 11" latex balloon filled with helium is about 10g (0.35 oz), and of an 18" round foil balloon 3g (0.1 oz). That means the weight for your base needs to have at least 23g (0.55 oz).
As you are using 5" balloons for the base, you could fill one or two of them with a bit of water or sand to make them heavier. Or you glue the bottom balloons to a round foam base. Alternatively, you could purchase one of the many ready made balloon bases.