Before I forget, I'm not sure if making a rack wide enough for 3 ten gallon aquaria is a good idea. You might get some sagging. That's a shame as three wide would be good use of material. However, since you'll be using a 1/4" gap, any sagging won't have the undesired effect of cause the tanks to stick in the rack.
I would experiment with this. Cut a piece of melamine to the size you'll need and rest it between two books or something. Have just the ends of the melamine barely on the books. Then place three tanks with substrate, water bowls, everything. Leave them there for a day or two and see if the middle of the melamine is sagging.
Light rope varies in wattage per foot, and racks vary in several factors. Hard to say how much you will need for your rack.
But don't worry, it is cheap.
Since it gives off light you'll need to turn it off at night. This is not a problem, IMO, unless the room the rack will be in gets excessively cold during the winter.
But even then there is colored light rope. You could have regular light rope for the day and red or blue for the night.
Back to how much. You'll need to experiement with how much it takes to heat your rack. If it's not getting hot enough run more loops of it. If your rack is too ventilated for the heat to build up you may need to run it in conjunction with some 3" heat tape.
But I'd avoid heat tape since you're already using something else. You might insulate the back of the rack. Make sure to use that strene insulation that has a layer of foil on it. The thick styrene helps block the heat that builds up in the back of the rack. The foil blocks most of the radiant heat which causes the heat to build up in the first place.
Sort of an insulation double whammy.