A single T5HO bulb will light the substrate directly under it to around 50-60 micromols of PAR, medium light, with that tank, but it takes two bulbs separated about 6-8 inches to light up the whole substrate reasonably evenly, at about that same light intensity. With two bulbs close together, you get high light directly under the bulb, unless you raise it to about 30 inches from the substrate. All of this with a light that has very good reflectors and a real T5HO ballast. If the light you have is a good one, just suspend it above the tank and you will get better spread of light, better unformity, and a good light intensity. No need to ever use a 3rd bulb, unless the light is a cheap one without good reflectors or ballast.