3 watts per gallon is typically considered high lighting, however the watts per gallon rule by itself is typically too inaccurate to apply (kind of like the inch per gallon rule for fish). The type of light, intensity of the bulb, height of the tank, clarity of the water and other factors will determine if your lighting is enough. Then other factors such as hardness, temperature and pH would also determine which types of plants you can keep. For a 5 gallon tank, I don't imagine it would be too difficult to really keep any type of plant you want but at 3 watts per gallon you should be fine with java moss and java fern.
most any plant should grow with that lighting, provided you have proper fertilizer, co2, and good substrate. Java fern and java moss should do fine, I grow both with 2.5 wpg, I would try to put them in the shade of the other plants or maybe somewhere out of direct light.
The light being high up over the tank will make a bit of a difference depending just how high it is. I'm afraid the WPG rule doesn't apply to tanks under 10gal so you will actually find yourself restricted on the sorts of plants you can grow. I'm no fan of the wpg rule but basically there is a minimum intensity under which a plant can perform photosythisis. Once you get down around those lighting levels, the tank size becomes irrelevant & all that matters is whether or not the light is intense enough for the plants that you want. 15w of light is a low light tank I'm afraid, regardless of what capacity the tank is. I strongly advise you to get the light you have as low as possible over the tank to maximise the amount of usefull light you have.