Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan.
In a medium bowl, sift together flour, baking powder and salt. Set aside.
In a large bowl, cream margarine and 2 cups sugar until fluffy. Add eggs one at a time, mixing well with each addition. Alternately add milk and flour mixture in three stages, mixing well with each addition.
In a small bowl, mix 3 tablespoons sugar with 1 teaspoon cinnamon. Sprinkle this mixture evenly over cake.
Pour into prepared pan and bake at for 35 to 40 minutes or until done.