In a mixing bowl, stir together flour and 3 tablespoons brown sugar. Cut in 1/2 cup margarine until mixture resembles coarse crumbs. Bake in an ungreased 11 x 7 x 1 1/2-inch baking pan at 375° for 20 minutes. In another bowl, use a fork to beat eggs. Stir in 1/2 cup brown sugar, pecans, corn syrup, margarine and vanilla. Pour over baked crust, spreading evenly. Bake 15 to 20 minutes more, or until set. Cool. Cut into bars. Chill to store. Makes 24.