What kind of plants are those?
I'm not exactly sure what you are tyring to do. The flood lights are more concentrated than the marineland. If you are trying to increase the over-all brightness of the tank, putting one on surface will give you a hot spot (3 may work). If the light is a foot or two above the tank It will give a wider spread but will also light the entire room if its not enclosed. If you have a shadow area you want to cover I'd go with a 10w. Even then trying to match the brightness of the floods to a marineland will be tough. Personally I like shadows in my tanks.