In heavy saucepan, dissolve cornstarch in water. Stir in condensed milk and egg yolks. Cook and stir until thickened and bubbly. Remove from heat. Add butter and vanilla. Cool slightly. Slice 2 bananas, dip in lemon juice and drain. Arrange on bottom of crust. Pour filling over bananas, cover. Chill 4 hours or until set. Spread top with whipped cream. Slice remaining banana dip in lemon juice and drain. Garnish top of pie with last banana. Refrigerate leftovers.