Decrease voltage but keep same Amperage?
This can't be done with a resistor. The voltage drop will change depending on the current.
It could sort of be done with diodes, but it will not work well and it will waste tons of power.
You either need a different power supply, or a SWITCHING REGULATOR, also known as a buck converter. These are available pre-assembled on a PCB:

https://www.aliexpress.com/item/4000064597454.html
Check the buck converter voltage and current specs to make sure it can supply 12V at 3A.
It will have to be a switching regulator, NOT something linear like the LM7812, as that will get extremely hot due to the inefficiency.