Likely your 12V batteries are NOT true deep cycle batteries and do not like being discharged at such a high rate. "Marine" batteries are "dual purpose", but really aimed at high starting currents. True deep cycle batteries alomost never advertise Cold Cranking Amps (CCA) or Marine Cranking Amps (MCA).
Are all 3 batteries the exact same make and model and purchased at the same time ? If not one of them may be pulling down the others. As soon as is reasonable after your load test, disconnect the batteries from one another and check the voltage at each battery post. More than a 0.10V difference is not good.
