The lighting wouldn't matter if the rest it home is unstable. And keeping the temp, salinity, calcium/ Alk/ Mag levels, ph, nitrate, nitrite, amon would be an very tedious task. I have to dose B-Ionic every day in my 50g system.
You would have to get a small clam no doubt and thought i've heard that small clams are way more dependent on Phyto to live and need frequently. Phyto can extremly polute water, and in a 3g tank would be very difficult. Why risk killing such a amazing animal by providing with a rediculously small tank for what its recomeded to be housed in.
The smallest recomended tank for any of the giant clams is 50g in Ronald Shimeks Marine Invert book. A 3 gallon tank would be 6% of the recomended volume.