Can the Rheem RTex06 water heater that is rated at 1.5 gal / min. be installed in series to heat 3.0 gal / min

Yes, two Rheem RTEX-06 water heaters can be installed in series to effectively heat a flow rate of 3.0 gallons per minute. Installing them in series means the incoming water is preheated by the first unit and then further heated by the second, allowing for greater temperature rise at higher flow rates.