Generally speaking you want a set number of watts for each gallon of tank size because it's an easy way to gauge having about the right intensity of light for any given tank size. For a low light, low growth tank (with slow growing plants) you want anywhere from 1 to 2 watts for each gallon of tank size (often 1 1/2 is a better place to start, though). For a Medium/Moderate light tank, you want between 2 and 3 watts and this is a nice area to be in for the simple fact that you can have pretty much any plant you want (with a few exceptions)and not be forced into using CO2 or expensive lighting, although many start needing at least a DIY CO2 system as they get towards 3 wpg. Then 3 to 4 wpg is necessary for a high light tank. This type of tank, almost certainly needs to be using supplemental CO2 and added fertilizers to keep up with growth demands. If you don't get the balance just right, algae is going to take over the tank and cause serious problems. Finally some have even used 4 watts or more for extreme bright lighting for very needy plants or very deep tanks. The reason is that water tends to break down the light wavelengths over distance, and filter out much of the necessary light. Many low growing high light requirement plants (like glosso) are grown as low foreground plants and will need intense lighting all the way to the bottom. This type of tank will be needing high or bright light tanks. That's the gists of it anyways.
As for your setup, I can't really comment on how much light you have because you didn't list the size tank you are using. Just be aware that Most 10 gallon tanks (standard) come with 15 watt T8 lighting, which is actually a good place to start a low to moderate planted tank. Even though it's only 1 1/2 watts per gallon, the shallow depth of this size tank is small enough to allow most light full depth access. They also use T8 lighting which is thinnner tubes which are more efficient at getting more light in the tank than the T12 normal flourescent tubes. Also, you said that you only have 3 plants. Now depending on the size of your tank, and what those 3 plants are, I'd be very tempted to say that your tank is less than even lightly stocked. The only time you would have decent amount of plant growth for your tank is if you had large growing plants like some Amazon Swords or such. If you have a couple of Anubias and a stem of Ludwigia, then even a 10 gallon isn't lightly stocked. I wouldn't worry about considering your tank a planted tank until at least 1/3 of it is planted (personally I'd call that lightly stocked with plants). Most people either have "fish tanks with a few plants" or "planted tanks with a few fish". See the difference? Hope I helped a bit.
