I Replaced Pressure Relief Valve on Rinnai water heater (twice!!). New one does not hold ANY pressure
By any chance had you turned on the water heater BEFORE turning on the cold water supply to it? Because if so, you may have flashed the water to steam instantly, which is what the PRV is there to protect against. The same could have happened if you had not opened a hot water valve somewhere, because the heater would have had trapped air in it and very little water in the chamber, so it again flashed to steam and triggered the valve(s). If on the third try you turned on the water, had a faucet on so that the air flowed out of the heater, THEN turned on the heater, it would have acted normally to heat the water at the designed rate; no flashing to steam, no excess pressure.