Reduce voltage of DC adapter without reducing current

I have instant hot water heaters which take 2 X 1.5 batteries = 3V. I
want to replace the battereies with DC adapters we have. The adapters
have various voltages. I tried to drop the voltage by adding a resistor
to the circuit, but then the current drops too low for the unit to fire
up. Is there anything I can do? The measurements are:

Batteries: 3.36V, 22.2mA

DC Adapter, no resistor added: 5.9V, 22mA (Why does the current not
increase with the increased voltage?)

DC Adapter, resistor added: 5.9V, 15 mA and unit doesn't fire.

One unit is running OK using the DC adapter without a resistor, but I
think it would be risky to use the higher voltage adapters without
reducing the output voltage to near 3V.

.

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