It depends on how the CO2
is added to the tanks, and how successful that method is, whether the three tanks can be run off the same sump. Experiment-wise, it would be better for all three tanks to run off the same sump, but I would set up the whole thing and test to make sure the CO2
levels in the tanks are what you want them to be before putting the clams in....if you can maintain the levels you need on one sump, then you're all set. If not, then 3 separate systems, but you're going to have to be really careful to address conditions so that you don't get a confounding variable.
Personally I think 3 -33 gallon long tanks, run to the same sump would be best. If they ahve separate lighting systems you need to check that the amount of light reaching the bottom of each tank is the same (use a PAR meter). if the tanks are close together then it might be necessary to put dividers between them so that the middle tank doesn't end up with more light than the end ones. My lab uses MHs for experiments and we have to shield them because of this.
I am by no means a clam expert but I am a 5th year oceanography PhD student so please ask any and all questions and I will do my best to answer them!